Doping for making n- and p-type a-GaP:H films during reactive evaporation and sputtering

1OUR N A L OF

Journal of Non-Crystalline Solids 137&138 (1991) 935-938 North-Holland

NON-CRY LINESO

DOPING FOR MAKING N - A N D P-TYPE A-GaP:H FILMS DURING REACTIVE EVAPORATION AND SPUTTERING Takanori FUJIYOSHI, Masami ONUKI', Kenji HONMYO, Hiroshi KUBOTA and Tsuyoshi MATSUMOTO Department of Electrical Engineering and Computer Science, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860, Japan "Department of Commercial Sciences, Chuo-Gakuin University, 451 Kujike, Abiko, Chiba 270-11, Japan Techniques of doping with zinc and sulfur to a-GaP:H films during hydrogen-reactive evaporation (RE) and sputtering (SP), and properties of p-n junction diodes made of different a-GaP:H films are described. The conductivity of a-GaP:H films, made by RE and SP method, increases drastically by doping. Junction diodes composed of p-type RE /or SP a-GaP:H films doped with Zn on n-type crystalline GaP substrates, show a comparatively good rectification property, while that of a diode of (p-type SP film doped with Zn)/(n-type RE film doped with S) is in bad shape at present. On the other hand, the latter diode shows the photo-voltaic effect, the spectrum of which is compared with the photoconductivity one of RE a-GaP:H films reported by us previously.

1. INTRODUCTION An amorphous hydrogenated gallium phosphide (aGaP:H) is considered to be a useful material for optoelectronic devices working in the spectral region from green to blue, since the energy gap of a crystalline Gap is 2.26 eV at room temperature and that of a-GaP:H should be higher than it. On the other hand, some difficulties are there in developing devices of amorphous compound semiconductor, which are caused by localized states due to dangling bonds and chemically wrong ones. In our recent works 1~'2~, some properties of the states which interrupt the electronic transport in the material have been made clear with the proposal on the band scheme of a-GaP:H. Furthermore, electro-luminescence was observed for a diode composed of an n-type aGaP:H film made by the reactive evaporation (RE) without doping, on a p-type crystalline GaP (c-GaP) substrate 3), where the emitting light was green but observed only in the reversed bias2). Now, light emitting diodes in the visible region, made of all of the amorphous III-V semiconductor film have been desired for different uses. In this paper, techniques of doping with zinc and sulfur to a-GaP:H film, during the reactive evaporation (RE) by using hydrogen ion gun/or the sputtering (SP)

in mixed gases of hydrogen and argon, are reported. The current-voltage (I-V) characteristics for different diodes are described, which were made of doped aGaP:H films by RE and SP methods on n-crystalline substrate. In order to evaluate conductive a-GaP:H films applicable to practical light emitting diodes, the resistivity of different amorphous films doped with zinc or sulfur was measured by using the four probe method. A good I-V characteristics has been obtained for a diode of (p-type SP a-GaP:H film doped by using sintered zinc phosphide)/(n-type c-GaP substrate). On the other hand, the I-V characteristics of a diode of (Zn-doped SP a-GaP:H film)/(S-doped RE a-GaP:H film) did not show a definite rectification, while the photo-voltaic effect was observed in the diode under illumination. The spectrum of it is compared with the photoconductivity spectrum reported by us 1~. 2. DIODES COMPOSED OF Zn-DOPED A-GaP:H FILMS ON N-TYPE C-GaP SUBSTRATES Different p-type films of a-GaP:H by RE and SP methods were deposited on n-type crystalline GaP substrates, 5ram x 5mm in area. The evaporation source powders in RE method were prepared as follows; a lot of small broken pieces, about 100 I~m in

0022-3093/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved.

936

T. Fujiyoshi et aL ~Doping for making n- and p-type a-GaP: H films

dimension, of p-type crystalline GaP, mixed with powders of zinc m e t a l / o r zinc phosphide, was sintered at 300 °C for 5 hours and then pressed into a pellet, about 10 mm in diameter and 3 mm in thickness, under a pressure of 5 tons for ten minutes. The pellet was annealed again at 500 °C for an hour and ground into powders. In the case of SP method, the above pellet or a wafer of single crystalline GaP wafer with zinc powders was used for a target. For electrodes of the diodes, gold or gold/zinc was used for p-type films and crystals, and gold or gold/tin for n-type ones, in applying evaporation technique. The condition in depositing different films on n-type crystals for making different diodes and the mole ratio of Ga : P : Zn estimated by EPMA are tabulated in Table 1. The I-V characteristics for different diodes of (Zndoped RE a-GaP:H film)/(n-type c-GaP substrate) and (Zn-doped SP a-GaP:H film)/(n-type c-GaP substrate) are shown in Figures l(a) and l(b), respectively. Curve 1 shows the I-V characteristics for a diode made of a RE film without dopants, which shows non-rectification. In general, RE films without dopants are slightly of ntype, as explained in the previous paper 1)'3).Curve 2 and 3 correspond to the diodes made of RE films with a zinc dopant, where the annealing process in making the evaporation source material was omitted for the sample of Curve 2. As seen in Curve 3 of Figure l(a), p-type R___E_Ea-GaP:H films have been possible to obtain by doping with zinc. Figure l(b) indicates the I-V characteristics of diodes made of (SP a-GaP:H film)/(n-type c-GaP substrate),

1.0

31

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2

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1

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Figure l(a) I-V characteristics of diodes of (Zn-doped RE a-GaP:H films)/(n-type crystalline GaP substrate)

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Table 1 Preparation condition and mole ratio of Ga : P : Zn. . -1.0 i sarnple p r e p a r a t i o n _H ~

m o l e r a t i o (Ga:P:Zn)

0.15:1:0

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0.45:1:0.025 0.24:1:0.027

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Figure l(b) I-V characteristics of diodes of (Zn-doped SP a-GaP:H filrns)/(n-type crystalline GaP substrate)

937

T. Fujiyoshi et aL ~Doping for making n- aMp-type a-GaP.'H films

300K

where Curve 4 and 5 correspond to ones without and with a zinc dopant, respectively. Although films deposited by SP method are slightly of p-type even without a zinc dopant, the conductivity is concluded to be enhanced very much by doping a Zn dopant, comparing the forward current in Curve 5 with that in Curve 4.

3

Since higher conductivities of the n- and p-type films are essential to obtain amorphous diodes exhibiting an

0.5

FORWARD~

v

excellent I-V characteristics, the dark resistivity of different films deposited on glass substrates were measured by using a four probe method. The resistivity of RE films doped with zinc or sulfur is as low as 5 x

-5 w

,

I

I

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5

V(V)

10- 2 0 . c m at room temperature, while that without a dopant shows higher values than 10 f~.cm. The resistivity of p-type films withdopants was enhanced to about ten times as high as the initial one by annealing at 300 °C for several hours. 3. D I O D E COMPOSED OF Zn-DOPED SP A-GaP:H FILM ON S-DOPED RE A-GaP:H FILM A diode of (Zn-doped SP a-GaP:H film)/(S-doped

Figure 2 I-V characteristics of diode of (Zn-doped SP a-GaP:H film)/(S-doped RE a-GaP:H film)

RE a-GaP:H film) was fabricated on the glass coated with ITO, where the p-type film was in contact with a gold electrode and the n-type film in contact with an ITO one. The thickness of the former film was 800 nm, and that of the latter 150 nm. The I-V characteristics of

• 300 77K

.................................. '

A

the diode is shown in Figure 2, which is not in a definite shape, but presents a sign of rectification as seen in the first quadrant for comparison. In future, a diode with more excellent characteristics may be possible to make by carrying out different combinations of above film techniques.

I O

The spectral response of the ac photo-voltaic effect under a chopped illumination for this diode has been measured in order to reaffirm the band scheme previ-

i 1.6eV

ously presented 1). The illumination was incident to the glass substrate. The results are shown in Figure 3, where

I

the upper curve corresponds to that at room temperature and the lower one to that at 77 °K. In the lower curve, the peak in the photo-response is observed at 1.6, 1.9 and 2.3 eV, respectively. They are nearly consistent with the singularities in the response spectrum of the

1.5

1.9eV

2.3eV

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2.0 Photon

2.5 Energy

3.0 (eV)

Figure 3 Spectrum of photo-voltaic response for the diode shown in Figure 2.

938

T. Fujiyoshi et al. ~Doping for making n- and p-type a-GaP:H films

photoconductivity1). In Figure 3, the band at higher energies than 3 eV may be due to carriers made in the ITO electrode.

definite at present, but the photo-voltaic effect was observed for the diode, which means the formation of

We shall summarize the above results : 1) Different methods to fabricate doped a-GaP:H films to apply junction diodes are described. The hydrogen-reactive evaporation and sputtering in hydrogen gas are applied to deposit thin films, with dopants of zinc and sulfur. 2) The I-V characteristics of diodes composed of Zn-doped

of the film techniques.

a-GaP:H films deposited on n-type crystalline GaP substrates are shown. Some of them show a definite shape. 3) The rectification curve shape of a diode composed of all of the a-GaP:H film is not necessarily

2. M. Onuki, T. Fujii and H. Kubota, J. Non-Cryst. Solids 114 (1989) 792.

a p-n junction. In future, the I-V characteristics is possible to be improved by trying different combinations

REFERENCES 1. M. Onuki, K. Tsubusaki and H. Kubota, J. NonCryst. Solids 97&98 (1987) 1347.

3. H. Kubota and M. Onuki, J. Non-Cryst. Solids 115 (1989) 39.