1960 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 38, NO. 8 , AUGUST 1991

Electron-Beam-Induced Current Studies of (i) pcSi : H/ Crystalline (n+-p) Si

Heterostructures

G. C. Dubey, R. A. Singh, S. K. Aggarwal, B. L. Sharma, and A. K. Sreedhar

Abstract-Electron-beam-induced current (EBIC) studies of intrin- sic, highly disordered, or nearly amorphous Si : H thin-film-deposited (n'-p) single-crystal silicon solar cells have been reported in this brief. The results are compared with TiO, deposited solar cells.

I. INTRODUCTION

During the past few years, considerable interest has been aroused in using heterostructures of amorphous and crystalline Si for solar cells [ l j , [2]. As reported earlier [3], i t has been observed that short-circuit current of a space quality n+-p-p+ Si solar cell can be substantially improved by depositing an intrinsic layer of pcSi : H on top of it. In order to have an insight into the mechanism, EBIC studies in this type of heterostructure were undertaken. In this brief the EBIC results obtained for the above structure are reported and compared with those having a TiO, layer instead of pcSi : H layer on the silicon solar cells.

11. EXPERIMENTAL

The crystalline Si structure used to form heterostructures for this study consisted of either space quality n+-p-p+ Si or texturized n + - p Si terrestrial solar cells. The former cells were made by diffusion of phosphorus at the front and aluminum at the back of a 2 4 . cm p-type Si wafer while the latter by diffusing phosphorus after tex- turization with NaOH solution of a 3 - 5 4 * cm p-type Si wafer. The top contact fingers and a large-area back contact were made in the former case by depositing three-layer Ti /Pd/Ag and four-layer Al /Ti /Pd/Ag structures, respectively, while in the latter case by screen printing of pure silver paste and silver aluminum paste. The structure used in this study was formed by depositing an intrinsic pcSi : H layer of thickness 200-300 A by RF glow discharge using 1 % Si€& and 99% H, at a temperature of 150°C [4] on the solar cells. The optical energy gap of these films was found to be about 1.7 eV and the measured refractive index (ellipsometer model 116B, Gaertner Scientific Corp., USA) was between 3.8 and 4.1. These results and electron diffraction studies indicated that films were highly disordered or nearly amorphous. The typical short-circuit current and open-circuit voltage before and after deposition of pcSi : H layer on top of the texturized solar cells are given in Table I. The TiO, deposited solar cells for comparison were formed by the vacuum evaporation technique.

For EBIC measurements, the samples were mounted in a JSM 840A SEM and scanned from the p side to the n side with an elec-' tron beam of fixed voltage and current and a spot size of about 300

Manuscript received April 3, 1990; revised November 15, 1990. The

The authors are with the Solid State Physics Laboratory, Delhi-I 10054,

IEEE Log Number 9144399.

review of this brief was arranged by Associate Editor M. D. Feuer.

India.

A. The electron-beam-induced current was measured across the structure by Keithley Electrometer No. 610C and plotted on an X-Y recorder. It should be noted that in the case of texturized struc- tures a proper in situ rotation of the sample prior to every mea- surement was needed to adjust the direction of the movement of the beam spot perpendicular to the surface of the sample.

111. RESULTS A N D DISCUSSIONS

To explain the experimental results, two configurations shown in Fig. l(a) and (b) are considered. When a thin layer of (i) pcSi : H layer is deposited on a crystalline n+-p Si structure, the hetero- structure can be treated as having two junctions (i.e., one Schottky, due to the presence of a thin layer of (i) pcSi : H between the metal and the n f Si [5], [6]; and another n f -p homojunction). The equi- librium energy band diagram of the configuration of Fig. I(a) is shown in Fig. 2 . It can be seen from this figure that the built-in voltage VD, between (i) pcSi : H / n + Si heterojunction is greater than VD2 of the n t7p Si homojunction. For example, the calculated values of VD, and VD2 for typical carrier concentrations are 1.45 and 0.93 eV, respectively [3]. In order to confirm the above men- tioned two-junction model, samples having Fig. l(a) configuration (i.e., (i) pcSi: H layer deposited on top of n+-pjunction) were also fabricated. The EBIC plot of such a heterostructure is shown in Fig. 3. The two observed peaks can be explained on the basis of a two-junction model as follows: when electron-hole pairs are gen- erated in the vicinity of n+-p junction of the p side, the electrons cross the junction and accelerate towards (i) pcSi : H-n+ Si junc- tion. A part of these electrons, on approaching the highly disor- dered or nearly amorphous layer, cross the junction by hopping between the gap states near the Fermi level and/or tunneling [7], [8j and contribute toward the smaller peak appearing in the EBIC plot. Since the generation of carriers near the Shottky barrier, by illumination or electron beam, has the same effect as applying a forward bias to it [9], more and more electrons cross the Schottky barrier as the electron-beam spot approaches it. The generation of electron-hole pairs in the vicinity of this Schottky junction and most of the electrons crossing it is responsible for the second higher peak in the EBIC plot.

When EBIC measurements were carried out on the configuration shown in Fig. l(b), some interesting features were observed. If the electron-beam spot was kept at position I (i.e., in front of the metal finger contact) and scanned along the arrow, only one peak in the EBIC plot similar to the one in n+-p Si. single-crystal solar cell was observed. When the electron-beam spot was kept at position I1 (i.e., slightly away from the metal finger contact) and then scanned along the arrow, the EBIC plot showed two peaks similar to Fig. 3 . By shifting the electron-beam spot to a position far away from the metal finger contact it again showed one peak. This could possibly be due to the fact that the electrons entering the films are not able to reach the finger contact. This observation could, therefore, be qualitatively explained on the basis of the above mentioned two- junction model, diffusion length, and lifetime in n + Si and pcSi : H and electrons following the least resistance path.

In order to show that the substantial increase in short-circuit cur- rent of these solar cells is due to the two-junction model, the EBIC studies of TiO.,-deposited solar cells were carried out. The EBIC plots in this case showed only one peak as observed in uncoated solar cells in all positions.

0018-9383/91/0800-l960$01 .OO 0 1991 IEEE

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 38, NO. 8. AUGUST 1991 1961

TABLE 1 THE PARAMETERS OF T H E TEXTURIZED si SOLAR CELLS (LOO-” DIAMETER) BEFORE A N D AFTER COATING

OF pcSi : H FILM

Before Coating After Coating

Cell I , < I,,,,( V = 450 mV) v,, I , , I,,/,( V = 450 mV) v,, Number (A) (A) (VI (A) (Ai (VI

I 2.38 2.04 0.57 2.46 2.12 0.57 3 2.45 2.23 0.57 2.71 2.48 0.58 I 2.42 2.26 0.57 2.72 2.49 0.57

54 2.45 2.23 0.57 2.68 2.47 0.58

LARGE AREA HETAL LAYER

(a)

- METAL CONTACT

l i t pc Si:H LAYER

(b)

Fig, I . Schematic representation of two configurations. (a) Metal contact taken from ( i ) pcSi: H film. (b) Metal contact on the solar cell.

METAL ( i ) p c S i : H ( n ’ ) s ; ( P ) si Fig. 2. Equilibrium energy-band diagram of metal-pcSi : H-n’Si-p Si

structure.

u c SiH

RELATIVE DISTANCE (a rb . units) - Fig. 3. EBIC plot of metal-pcSi : H-n+ Si-p Si structure.

IV. CONCLUSIONS

The presence of two peaks in the EBIC plot and their relative heights gives a clear indication that two junctions (i.e., one hete- rojunction and another homojunction) are operative in these het- erostructures. The earlier explanation for the reported short-circuit current enhancement observed in solar cells coated with (i) pcSi : H layers, therefore, seems valid. The higher values of measured re- fractive index of the film and similar results on both plane and texturized Si solar cells also suggest that this enhancement is not due to an antireflection coating effect. Although an enhancement in short-circuit current is also observed when the solar cells are coated with TiO,, it is mainly due to better optical matching.

REFERENCES

[I] K . Okuda, H. Okamoto, and Y . Hamakawa, “Amorphous Siipoly- crystalline Si stacked solar cell having more than 12% conversion ef- ficiency,” Japan. J . Appl . Phys. (U), vol. 22, p. L605. 1983.

[2] M. M. Rahman and S. Furukawa, “Amorphousicrystalline hetero- junction as a novel approach to fabrication of a solar cell,” Electron. Lett., vol. 20, p. 57, 1984.

[3] B. L. Sharma, G. C. Dubey, and V. K. Jain. “New type ofhetero- structure for solar cells,” in Physics qf Semiconductor Devices. S. C . Jain and S. Radhakrishan, Eds. Singapore: World Scientific. 1987, pp. 445-462.

1962 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 38, NO. 8. AUGUST 1991

141 G. C. Dubey, R . A. Singh, S . N. Mukerjee. S . Pal, and M. G. Rao, “Electrical and Optical properties ol‘pcSi : H films,” Bull. Muter. Sci. , vol. 8, p. 267, 1986.

[5] B. L. Sharma and R. K . Purohit, “The variation of Schottky barrier height due to an interfacial layer,” Surfircc, Tcdiriol . , vol. 1 I . p. 41 I , 1980.

161 S. Lovaliche, C. Vandry, L. Henry, and A. Lecorre, “ a : Si : H Schottky gate on n-GaInAs,” Elecrrorz. Lett.. vol. 22, p. 896, 1986.

[7] B. L. Sharma, “Ohmic contact to 111-V compound semiconductors,” in Seniiconductors uiid Scwzitiietrrls, vol. 15, R . K. Wilardson and A. C. Beer, Eds. New York: Academic, 1981, p. 8.

[8] T. Sebestysen. “Model of a possible class of ohmic contacts to semi- conductors based on a graded amorphous-crystalline junction.” in Amorphous Semiconductor 76, 1. Kosa Somogy, Ed. Budapest, Hun- gary: Akademiaw Kiado, 1977, pp. 321-326.

[9] G . J . Chaturevedi, R . K . Purohit. and B. L. Sharma, “Optical effects in GaAs MESFETs,” Iniji-urd Phps.. vol. 23. p. 65, 1983.

Effective Acceptor Profiles in Chemically and Plasma Treated WN,/GaAs Shannon Contacts from

Capacitance and Current Measurements

P. E. Bagnoli, A. Paccagnella, and A. Callegari

Abstruct-This study concerns WN, /GaAs Schottky diodes fabri- cated using three semiconductor surface cleaning procedures and three temperatures for the post-metallization annealing. The electrical char- acteristics were interpreted on the basis of the enhanced barrier Shan- non contact theory, assuming that the surface p-type dopant is pro- vided by atoms implanted during the sputtering process or diffused during the annealing. The parameters of the effective acceptor profiles, which are the surface concentration, the maximum penetration depth, and the total amount of acceptors per unit area, were calculated by means of a method where the voltage intercept of the capacitance curve and the ideality factor of the forward current characteristic are re- quired as input data. The enhancement of the barrier after the an- nealing strongly depends on the surface cleaning procedure before me- tallization.

I. INTRODUCTION

The barrier height of sputtered rcfractory metal nitride/GaAs contacts was found to be an increasing function of post-metalliza- tion annealing (PMA) temperature, up to 850°C [ I ] , 121. Specially in the range 7O0-85O0C, the diodes show a large decrease of the junction capacitance and a dramatic increase of the voltage inter- cept of the C-’-V curve (sometimes well above thc GaAs cnergy gap), together with an ideality factor of the currcnt-voltage ( A V ) characteristic very close to unity [2].

Manuscript received August 28, 1990; revised December 3 , 1990. This work was partially supported by C.N.R.-Progetto Finalizzato Materiali e Dispositivi per I’Elettronica a Stato Solido. The review of this brief was arranged by Associate Editor B. Ricch.

P. E. Bagnoli is with Instituto di Elettronica e Tclecomunicazioni, Uni- versita di Pisa, 56100 Pisa, Italy.

A. Paccagnella is with the Dipartimento di Elettronica e Informatica, Universita di Padova, 35131 Padova, Italy.

A. Callegari is with IBM Thomas J . Watson Research Center. Yorktown Heights, NY.

IEEE Log Number 91001 12.

Since the sputter-induced damages near the GaAs surface gen- erally produce a lowered barrier height and a worse ideality factor [3]. [4], the presence of these traps cannot be invoked for explain- ing the behavior of the diodes annealed at the higher temperature range, although the problem is still debated.

Zhang et al. [2] have proposed that the annealing in the range 500-700°C causes a recovery of the sputter-induced damage. For the higher temperature range the increase of the barrier height and the decrease of the junction capacitance have been explained by postulating the formation of a shallow p f / n contact (i.e., Shannon junction), where the p layer is provided by atoms implanted during the sputtering process or diffused within the semiconductor during the annealing process. In particular, the acceptor-like shallow im- purities have been ascribed to nitrogen-defect complexes and/or nitrogen atoms [2].

Although there is no direct evidence that nitrogen creates accep- tor states in GaAs, the above hypothesis is also supported by the fact that sputtered WSi,/GaAs diodes, fabricated without nitrogen in the metal deposition process, do not show barrier height increase when annealed at higher temperatures [ 5 ] , [6]. For this reason, in the present brief, we followed the hypothesis of the p’/n Shannon contact formation, in order to study the electrical properties of tungsten nitride/GaAs diodes, fabricated using three surface cleaning procedures and three PMA temperatures in the range 700- 850°C.

It was found that the enhancement of the barrier height after the PMA strongly depends on the surface preparation before metal deposition. The effective maximum penetration depth and the ef- fective surface density of acceptors were evaluated using the con- ventional current-voltage (J-V ) and capacitance-voltage (C-V ) measurements, interpreted in terms of the enhanced-barrier Shan- non contact theory [7], [81.

11. THEORY

For the analysis of the experimental results, we assume that in our samples the thickness of the pf doped layer is much smaller than the depletion-layer width ( w ) of the Schottky junction, so that the acceptors are fully depleted.

Since in the present case the acceptors can be due to atoms im- planted or diffused from the surface, a distribution decreasing in depth is expected. Therefore, we assume the acceptors distributed as a ramp, decreasing from the surface up to an effective maximum penetration depth L , as shown in inset a of Fig. 1 . NAo (cm-’) represents the effective acceptor density at the semiconductor sur- face and ND is the donor density in the bulk of the semiconductor.

Solving the Poisson equation, the energy potential E , ( s ) (i.e., the bottom of the conduction band) (see inset b of Fig. I ) can be calculated taking into account the effect of the image force lower- ing.

q NAo L’ (x - L)3 + ~

4 ’ NA” i,, + 6E,L 6Q

for L < x < w

0018-9383/91/0800-1962$01.00 0 1991 IEEE