Enhancing optical absorption in InP and GaAs utilizing profile etching

The current state of profile etching in GaAs and InP is summarized, including data on novel geometries attainable as a function of etchant temperature, composition, and rate, substrate orientation and carrier concentration, and oxide thickness between substrate and photoresist. V-grooved solar cells have been manufactured with both GaAs and InP, and the improved optical absorption demonstrated. Preferred parameters for various applications are listed and discussed.<<ETX>>

Prepared for the 22nd Photovoltaic Specialists Conference sponsored by the Institute of Electrical and Electronics Engineers Las Vegas, Nevada, October 8-11, 1991 INTRODUCTION Profiling techniques have played a large role in increasing efficiencies in silicon solar cells in a variety of ways. Short circuit current is increased because the reflected incident light is redirected to the surface. Other advantages include enhanced absorption [1], light trapping [2], reducing grid shadowing [3], and advanced junction/contact geometries [4,5]. The extension of these techniques to the III-V materials has been slow, principally because anisotropic etching in the III-Vs is more complicated than on silicon [6]. The (111) plane is chemically different from the (ITT) plane, and both etching and deposition behave differently on these surfaces.

ANISOTROPIC ETCHING
The process of etching profiles in III-V materials requires (i) noting type and carrier concentration of substrate, and oxide layer between photoresist and substrate, (ii) masking the surface with photoresist stripes, aligned to the required crystallographic direction, (iii) selecting an appropriate anisotropic etchant, and (iv) carefully monitoring etch conditions such as time, temperature, and illumination. In compound semiconductors such as GaAs and InP, the [01T] and [01T1 alignments on the (100) surface are non-identical, and grooves oriented along [01T] will etch with significantly different profiles than grooves oriented perpendicular to this direction. It is important to note that there are two inconsistent conventions for the flat location on InP (100) wafers. Fig. I illustrates two possible geometries in both GaAs and InP with reference to a typical wafer alignment.
The conditions required to produce V-groove solar cells (see Fig. 2) in both GaAs [7] and InP [8] require careful control of the lateral undercut etch rate with respect to the vertical etch rate. The objective is to produce grooves without a flat plateau at the peak. This requires that the photoresist mask be undercut by the etch. For the solar cells produced here, the mask lines are --4 microns in width, and the groove periodicity was 7 to 8 microns. The wafer orientation is (100).
A variety of anisotropic etchants can be used with GaAs [9] and InP [10], however, only limited etchants and conditions will produce the triangular saw-toothed structure. When the photoresist lines are parallel to the [O1 T] direction GaAs can be etched to a perfect saw-tooth by the use of a Caros etchant, consisting of 5:1:1 proportions of H 2 SO 4 :H 2O 2: H 2O, respectively. At a temperature of 24°C with a native oxide layer between substrate and photoresist, the required etchant time is two minutes [11]. Anisotropic etching in GaAs is insensitive to type of dopant and carrier concentrations, however, etching rates can be altered by varying the oxide thickness between photoresist and substrate. When the mask lines are oriented perpendicular to the [011] direction, different crystalline planes emerge. In both GaAs and InP the undercutting is much more rapid. The etch profile in InP is again sensitive to the substrate dopant concentrations. Fig. 6 and Fig. 7 illustrate this effect. Fig. 8 shows the comparative structure in GaAs, which is insensitive to dopant type or concentration. In GaAs the orientation results in very clearly defined "hex" shaped grooves [6].

DISCUSSION
The reflectivity as a function of wavelength for ARcoated planar and V-grooved GaAs is illustrated in Fig. 9.
The v-grooved surface shows a significant reduction in the total reflectivity over the solar spectrum.
Similar results have been obtained in InP [8]. Fabrication of V-groove solar cells has been limited; however, results for GaAs, as illustrated in Fig. 10, indicate the potential rewards of developing such cells.
InP V-groove cells have been fabricated by means of closed ampoule diffusion [8], indium tin oxide (ITO) deposition and hydrogen plasma. In all cases an increased short circuit current density is realized with the V-groove.   . Both GaAs and InP exhibit similar gains in short circuit current utilizing the V-groove structure [7,8]. The advantages produced by the V-grooved structure are: reduced surface reflection due to double-reflection of normally incident light., enhanced absorption produced by oblique passage of light through the cell, improved radiation tolerance due to carriers being produced closer to the junction, and the possibility of manufacture of ultrathin, light-trapping structures with high performance and radiation tolerance. Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to