electrical properties of evaporated polycrystalline ge thin-films

E L S E V I E R Thin Solid Films 300 (1997) 138-143

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Electrical properties of evaporated polycrystalline Ge thin-films

Hiroald Kobayashi a,*, Narumi Inoue b, Takashi Uchida a, Yoshizumi Yasuoka a a Department of Electronic Engineering, The National Defense Academy, 1-10-20 Hashirimizu, Yokosuka 239, Japan b Department of Electrical Engineering, The National Defense Academy, 1-10-20 Hashirimizu, Yokosuka 239, Japan

Received 30 August 1996; accepted 9 October 1996

Abstract

Electrical properties of Ge thin films evaporated on Si3N 4 CVD-coated Si substrate were improved by introducing a heat treatment after the deposition of Ge films. Evaporation conditions were optimized by changing the substrate temperature and deposition rate, and then, heat treatment was performed. At substrate temperatures during the evaporation lower than 300 °C and higher than 400 °C, deposited films were amorphous and polycrystalline, respectively. At substrate temperatures lower than 400 °C, Ge films were evaporated without degrading the surface roughness. The Hall mobility of films evaporated at room temperature increased with increasing the substrate and heating temperature and showed about 400 cm 2 V -1 s -1 for the hole concentration of 4 × 1017 cm -3 at the heating temperature of 900 °C. This value was almost comparable to that of p-type Ge single crystal. © 1997 Published by Elsevier Science S.A.

Keywords: Heat treatment; Evaporation; Germanium

1. Introduction

There has been growing interest in Ge thin films on insulating substrates from the view point of material science and device applications [1-3]. Ge material has many advantages, compared to Si material, such as a relative large mobility and easily obtainable ohmic contact, and so on.

Thinking about device applications, the Ge thin film is one of the most important materials for fabricating antenna-coupled warm carrier device which detects infrared (IR) and far-infrared (FIR) laser radiation [4,5]. The warm carrier device consists of an antenna and a semiconductor detector. The two-dimensional device structure using a thin film semiconductor and antenna is necessary in order to obtain stable characteristics. Furthermore, the responsivity of the warm carrier device is proportional to the carrier mobility or the inverse of carrier density in the semiconductor material [6]. Therefore, the improvement of electrical properties of evaporated Ge films is important to realize a high performance antenna-coupled warm carrier device.

In this study, we aimed at investigating electrical properties of evaporated Ge thin films and optimizing evapora-

tion and heat treatment conditions to obtain good electrical properties of films.

2. Experimental set-up

In this study, a vacuum deposition method was used as one of the simple methods for obtaining Ge thin films on insulating substrates such as glass and Si3N 4 substrates. Ge films were deposited on Si3N 4 CVD-coated Si substrates (10 mm X 10 mm) by evaporating Ge powder (purity, 10 N). The thickness of the coated Si3N 4 layer was 150 nm. The pressures before and during the evaporation were less than 1 x 10 -6 Torr and t X 10 .5 Tort, respectively. In order to evaluate the crystal quality of films, we measured a structural property by X-ray diffraction and electrical properties, such as carrier density, resistivity and Hall mobility, by the Van der Pauw method at room temperature.

3. Experimental results and discussion

3.1. Evaporation of Ge thin-films

* Corresponding author.

First, we investigated structural and electrical properties of deposited Ge thin films by changing conditions such as

0040-6090/97/$17.00 © 1997 Published by Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 ( 9 6 ) 0 9 4 4 7 - 3

H. Kobayashi et a t . / Thin Solid Films 300 (t997) .!38-143 139

E -1-

104 - - 1020

10 3 . ~-~"1019

102. ~1018

101 ~ 1017

m i

0 1000

Tsub : 400°C

*--9

o

I I I I ~ I I I

500

Film thickness (nm)

10 o

10 -1 E

10-2._~ ._>

10-3

Fig. 1, Carrier density, resistivity and Hail mobitity of Ge films deposited on Si3N 4 at various film thickness. The subs~ate temperature was 400 °C for all samples.

1020 ~ ' ~ ' , ' = : 104 ~eposition rate:4Onm/min! 10 °

tFilm thickness :40Onto

# =; 10 2 1o-2._ > - E .~

"1-

101 10 -3

- 3 6 o ' 4 6 o ' 5 6 o '

Substrate temperature (%)

Fig. 3. Carrier density, resistivity and Hall mobility of Ge films at various substrate temperatures. The deposition rate and the film thickness were 40 nm min-~ and 400 nm, respectively, for all samples.

substrate temperature, deposition rate and film thickness in order to optimize evaporation conditions.

The electrical properties of deposited Ge films as a function of deposited film thickness are shown in Fig. 1. The substrate temperature was kept at 400 °C. All samples showed p-type conductivity and resistivities hardly depended on the film thickness. On the other hand, the carder density decreased with increasing film thickness and the carrier mobility increased with increasing film thickness. These values became constant at a film thickness over 400 nm. The Hall mobility showed about 30 cm 2 V - t s -1 for the hole concentration of about 2.0 X 1018 cm -3 at a film thickness over 400 nm. From these results, the deposited film thickness was chosen to be 400 rim.

Next, the electrical properties of deposited Ge films as a function of deposition rate are shown in Fig. 2. The substrate temperature and film thickness were kept to be 400 °C and 400 nm for all samples. All samples deposited

104 _ 100 10201: ' ' i . . . . . . . . i

:: Tsub : 400 °C " Film thickness : 400rim %-

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u p

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01 10 2

lO-1~" O

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r r

10-3

Deposition rate (nm/min) Fig, 2. Carrier density, resistivity and Hall mobility of Ge films at various deposition rates. The substrate temperature and the film thickness were 400 °C and 400 nm, respectively, for all samples.

at the deposition rate of 10-200 nm min -1 showed p-type conductivity and resistivities did not depend on the deposition rate. On the other hand, the carrier density and the carder mobility were almost independent of deposition rate up to 40 nm min -I. For the deposition rate over 40 nm min -1, the carder density increased with increasing the deposition rate and the carder mobility decreased with increasing the deposition rate. It was also found that the intensity of diffraction peaks of films increased with the increase of deposition rate from the measurement of X-ray diffraction spectra. From these experimental results, the deposition rate was chosen to be 40 nm min- t .

Finally, the electrical properties were investigated by changing the substrate temperature from 300 °C to 600 °C. These results are shown in Fig. 3. The film thickness and deposition rate were kept to be 400 nm and 40 nm rain -~, respectively. The resistivity and the carder concentration decreased with the increase of the substrate temperature and showed 0.05 f~ cm and 1.5 X 10 I8 cm -3 at the substrate temperature of 600 °C, respectively. The Hall mobility increased with the increase of substrate temperature and showed 100 cm 2 V - t s- ~ for the hole concentration of 1.5 X 1018 cm -3. It is generally known that, re- gardless of whether p- or n-type Ge crystal is used as a source material, vacuum-evaporated Ge films show p-type character on account of the existence vacancy acceptor levels [7]. The X-ray diffraction spectra of Ge films by changing the substrate temperature are also shown in Fig. 4. Although a weak (220)-oriented diffraction peak was observed at 300 °C, the deposited films were thought to be almost amorphous at the substrate temperature below 300 °C. The (111)- and (220)-oriented diffraction peaks were observed mainly at the temperature above 400 °C. The (111) orientation became gradually larger with the increase of the substrate temperature. On the other hand, the peak intensity of (220) orientation did not change with the temperature. This means that the main orientation begins

140 H. Kobayashi et aL / Thin Solid Films 300 (2997) 138-143

(111) (220)

j00o , 1 A 200 rt '-,4 klt

/ 400

200

20 30 40 50 60 2 0 angle (deg.)

Fig. 4. X-ray diffraction spectra of Ge films at various substrate temperatures.

Ge evaporat i on j Ge i ~ IS i3N4

1 ]~"-si Si02 sputtering [

Neat treatment l

I I SiOz etching l

Fig. 5. The processes of heat treatment with SiO z sputtering. After the annealing, the SiO 2 layer was removed by the wet etching.

to change from (220) to (111) with the increase of the substrate temperature. This is almost the same results as obtained by the X-ray diffraction measurement of Ge films deposited on a glass substrate [8].

From these results, it was found that the deposited Ge films tended to crystallize and these electrical properties were improved with increasing substrate temperature. And, evaporation conditions were optimized. The optimized evaporation conditions, substrate temperature of 600 °C, the layer thickness of 400 nm and the deposition rate of 40 nm rain-1 were obtained from the investigation of electrical properties of deposited Ge films.

3.2. Heat treatments of evaporated Ge thin fiIms

As mentioned above, it was found that a higher substrate temperature was required to obtain a better electrical property. However, it was difficult to increase the substrate temperature higher than 600 °C because of re-evaporation of deposited Ge films. Therefore, we introduced the heat treatment after the deposition of Ge films.

Processes of heat treatment are shown in Fig. 5. First, we deposited the SiO 2 layer, whose thickness was about 200 nm, on the deposited Ge films. Second, the Ge films covered with SiO 2 layer were heat treated in N 2 atmo- sphere. Then, the SiO 2 layer was removed by the wet etching using 20% HF. By introducing the heat treatment, the Ge films could be annealed up to 900 °C. It was confirmed that the film thickness did not change due to the heat treatment, the temperature of which was varied from 700 °C to 900 °C. On the other hand, it was also confirmed in our other experiments that the film thickness decreased from 400 nm to 100 nm by increasing the heating time from 0 to 60 min at the heating temperature of 900 °C without the SiO 2 layer. The heating time was optimized by measuring electrical properties and chosen to be 30 rain in the present experiment.

The electrical properties of Ge films as a function of

heating temperature are shown in Fig. 6. The substrate temperature during the evaporation was 400 °C. The film thickness was 400 nm and the heating time was 30 min. The electrical properties of films without heat treatment (without heating) are also shown in the figure as a comparison. All films heat treated at temperatures of 700-900 °C showed p-type conductivity and had resistivities of 0.06- 0.04 f~ cm. The carrier density decreased and the Hall mobility increased with the increase of the heating temperature. The Hall mobility of Ge film heat treated at 900 °C showed 250 cm a V -I s -1 for the hole concentration of 6 X 1017 cm -3.

The X-ray diffraction spectra of the films with the heating temperature (T a) from 700 °C to 900 °C are shown in Fig. 7. As it can be seen from the figure, the peak intensity of the (111) orientation of the sample with heat treatment is larger than that of the sample without heat treatment, and became gradually larger with the increase of the heating temperature. On the other hand, the (220)

: -~ "F ' ' ' ' ' ' 11°°.-.1

~Tsub : 400 °C 105 ~ 102°~ - Film thickness : 400nm

~" ! EHeating time • 30rain

• -~' 10 a 1018 10-2

,o-, 101 - 1 0 1 6 r wltl~out t p 700 800 9uO heating Heating temperature (°C)

Fig. 6. Carder density, resistivity and Hall mobility of Ge films at various heating temperatures. The substrate temperature was 400 °C, the heating time 30 rain.

H. Kobayashi et aI./ Thin Solid Fihns 300 (1997) 138-143 I41

(111)

~.J"~ 1 (200)

e,x..¢,~ "&'~" 900 JM ,~,.I , ~ (311)

,&,< ,e 800 , b - , J ~ +°

without heating

20 30 40 50 60 2 0 angle (deg.)

Fig. 7. X-ray diffraction spectra of Ge films at various heating temperatures+

orientation became gradually smaller with the increase of the heating temperature. The peak intensity of (111) has almost the same intensity as obtained from (220) orientation at the heating temperature of 800 °C and that of (111) orientation becomes larger than that of (220) orientation at 900 °C. These results show that (220) orientation changes to (111) orientation in polycrystalline Ge films with the increase of the heating temperature.

From the experimental results shown in Figs. 6 and 7, it was found that the structural and electrical properties of deposited Ge films could be improved by introducing the heat treatment.

The Hall mobility of Ge films as a function of substrate temperature are shown in Fig. 8. Three types of films are prepared in this experiment. The first type is Ge films only deposited on the substrate by changing the substrate temperature. The second and third types are Ge films heat treated at the temperature of 800 °C and 900 °C after deposition, respectively. As it can be seen from the figure, the Hall mobility of Ge films without heating increased

i , i , i e OTa 900 C,30mm

ATa 800°C,30rmn ©without heating

~" 10 a Deposition rate:40nm/min ~. Film thickness :400nm E

10 t I ~ " t I I

200 300 400 500 600 Substrate temperature (~C)

Fig. 8. The Hall mobility of Oe films against the substrate temperature was shown. Three types of Ge films were shown in the figure. One is the sample without heating and the others are samples heat treated at 800 °C and 900 °C, respectively.

with the increase of substrate temperature and that of films heat treated with the temperature of 800 °C increased with the temperature up to 400 °C and then showed the constant value above 400 °C. On the other hand, the Hall mobility of Ge films heat treated at the temperature of 900 °C did not depend on the substrate temperature and showed almost the constant value against various substrate temperatures. Ge films heat treated at 900 °C showed the highest value of about 250 cm 2 V-1 s - t in all samples.

Although Ge films deposited on the substrate below 300 °C showed almost amorphous behavior, these films became polycrystalline by introducing the heat treatment at 800 °C or 900 °C. However, many of Ge films deposited below 300 °C exfoliated from the substrate with the heat treatment because of the difference of thermal expansion between amorphous and polycrystalline Ge film.

From the results shown in Fig. 8, it was found that the improvement of electrical properties of Ge films without exfoliation from the substrate could be obtained by de- positing the layer at a temperature between 400 and 600 °C and performing the heat treatment at a temperature of 800-900 °C.

It is generally known that the carrier mobility of polycrystalline film depends on the number of barriers or barrier height between microcrystals in the film [9,10]. Therefore, we measured the grain size of Ge films in order to investigate the relationship between the carder mobility and crystal quality in detail.

The grain size of Ge films as a function of the substrate temperature are shown in Fig. 9. The grain size was obtained from the number of barriers in the Ge film estimated from the I - V characteristics [9] and the area of Ge film. Three kinds of Ge films were the samples without heating and with heating at 800 °C and 900 °C, respectively. As it can be seen from the figure, the grain size of films without heating increased with the increase of substrate temperature and that of films with heating at 800 °C increased with the temperature up to 400 °C and then

1000

E

.@ 500

t , i , L

OTa:900°C,30rnin ~kTa:800°C,30min O without heating Deposition rate:40nm/min FiJm thickness :400nm

O

O o . ~ . . . . . . . lO+ ~ ' ' °

0 P I

300 400 560 660 Substrate temperature (%)

Fig. 9. Grain size of Ge films against the substrate temperature was shown. Three types of samples were also shown in the figure.

142 H. Kobayashi et aL / Thin Solid Films 300 (1997) 138-143

showed a constant value above 400 °C. On the other hand, the grain size of films heat treated at 900 °C did not depend on the substrate temperature. These tendencies were almost the same with these of the Hail mobility of Ge films. It was found that the carder mobility of polycrystalline films depended on the grain size, namely the number of barriers between microcrystals, and large grains were effective for obtaining a large carrier mobility. On the other hand, carder concentration obtained by the Van der Pauw method shows the concentration in the grain itself [t0]. The reason why the Hail mobility increased with the increase of the substrate and heating temperature is thought to be the result of both the enlargement of gain size and the decrease of carder concentration in the grain because of the decrease of acceptor-like point defect [7]. However, the relationship between the barrier height and thickness is not clear at this stage.

The roughness of the Ge films was also investigated. The roughness of the sample as a function of the substrate temperature are shown in Fig. 10. The roughness of the Ge film was measured by die Talystep. Three kinds of Ge films were the samples without heating and with heating at 800 °C and 900 °C, respectively. All samples showed the same tendencies in the figure. Thinking about the samples without heating, it was found that the degradation of surface roughness occurred over the substrate temperature of 500 °C. Furthermore, even if the heat treatment were performed after the deposition, the degree of roughness was kept in almost the same condition with that of samples without heating. As far as the degradation of surface roughness was concerned, the substrate temperature during the evaporation was chosen to be below 400 °C. The surface roughness is very important concerning with the device application.

Up to here, the deposition rate was fixed to be 40 nm min -~. Finally, we measured electrical properties of Ge films heat treated at 900 °C by varying the deposition rate from t0 nm min -1 to 200 nm min -~. The results are

107

10 e

~i 105

104

10 s

- r 102

101

.~ 1 022! I A O Ta:900=O,30mln : . ,: ['qAO without heating • 102' ~ Tsub : 400 °C : ~ : Filmthlckness :400nm

I ~ ._ .# . . -~ . - . :~ . . . . rn ...-- ~oE 1020 r . - - . m - - . - - . L . - - . I . . . . •

L ;lo- t,, * - p - m1018 ~ . . . . . . . . .

. ,_ " ' - - ~

; 8to' L-" " -

I0 i

10 0

'i0-~ 0

10-2L

lo-3

10-4~

10-5 101 10 2 Deposition rate (nm/min)

Fig. 11. Carder density, resistivity and Hall mobility of Go films heat treated at 900 °C against various deposition rates were shown. Samples without heating were also shown in the figure as a comparison.

shown in Fig. 11. The electrical properties of films without heating were also shown in the figure as a comparison. As it can be seen from the figure, all samples heat treated at 900 °C had almost the same resistivities of 0.05 gl cm at the deposition rate of 10-200 nm min -1. On the other hand, the carder density and the Hall mobility decreased and increased with the increase of the deposition rate. This tendency of the Hall mobility was different from that of Ge film without heating. The Hail mobility of heat treated Ge film with the deposition rate of 200 nm min-1 showed about 400 cm 2 V -1 s -1 for the hole concentration of 4 X 1017 cm -3 and it was almost comparable to that of p-type Ge single crystal [11]. From the figure, it was found that a high deposition rate over 40 nm rain -1 was not effective to obtain a good electrical property for Ge films without the heat treatment, however, a high deposition rate was effective for films with the heat treatment at 900 °C.

4, Conclusions

S0 i I + i i

OTa:900°C,30min ATa:800°C,30min

40 (3without heating ~. Deposition rate : 40nm/min .g.= 30 Film thickness : 400nm

CO

¢ - A

20

rr"

1 0

u AO

o 260 360 460 660 Substrate temperature (°C)

Fig. 10. Surface roughness against the substrate temperature was shown. Three types of samples were also shown in the figure.

We have investigated some electrical properties of deposited Ge thin films and optimized evaporation and heat treatment conditions. Experimental results including optimized conditions obtained in this study are summarized as follows. 1. At a substrate temperature below 300 °C, Ge films were

almost amorphous and exfoliated from the substrate after the heat treatment. However, the improvement of electrical properties was obtained by the heat treatment.

2. At a substrate temperature of 400 °C, Ge films were polycrystalline and showed good electrical properties without degrading surface roughness after the heat treatment. The Hail mobility of 400 cm 2 V- t s-1 for the hole concentration of 4 X 1017 cm -3 was obtained under the conditions: deposition rate, 200 nm min-1; heating temperature, 900 °C; and film thickness, 400 r im.

H. Kobayashi et al . / Thin Solid Films 300 (1997) 138-143 t43

3. At a substrate temperature over 500 °C, Ge films were polycrystalline and showed good electrical properties after the heat treatment. However, the degradation of surface roughness occurred with the heat treatment.

References

[1] M. Takai, T. Tanigawa, K. Gamo and S. Namba, Jpn. Z Appt. Phys., 22 (1983) L624.

[2] J.E. Palmer, C.V. Thompson and Henry I. Smith, J. Appt. Phys., 62 (1987) 2492.

[3] N. Inoue, H. Kobayashi and Y. Yasuoka, Jpn. J. Appl. Phys., 3I (1992) L1266.

[4] K. Harakawa. Y. Yasuoka, K. Gamo and S. Namba, Jpn. J. Appl. Phys., 23 (1984) L203.

[5] N. Inoue and Y. Yasuoka, Trans. IEICE, C J-g9-c (1986) 571. [6] Y. Yasuoka and K. Harakawa, Rev. Laser Eng., 10 (1982) 200. [7] O. Weinreich, G. Dermit and C. Tufts, J. Appl. Phys., 32 (I961)

1170. [8] N. tnoue and Y. Yasuoka, Jpn. J. AppI. Phys., 27 (1988) 1437. [9] John Y.W. Seto. Z Appl. Phys., 46 (1975) 5247.

[10] Y. Yasuoka and T. Miyata, Jpn. J. Appl. Phys., 18 (1979) 1397. [11] S.M. Sze, Physics of Semiconductor Devices, John Wiley and Sons,

New York, 1981.

electrical properties of evaporated polycrystalline ge thin-films

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