M. Goradia

Electrolyte for electrochemical C-V profiling of InP- and GaAs-based structures

Abstract A new electrolyte (UNIEL) based on HF, NH3F2, C9H14CIN, CH3COOH and o-H3PO4 has been developed for accurate EC-V net majority carrier concentration profiling of InP- and GaAs-based III–V semiconductors. The new electrolyte was tested with good results on heterostructures containing p- and n-type InP, GaAs, InGaAs and InGaAsP layers.

Effective first layer antireflective coating on InP solar cells grown by chemical oxidation

Commonly used first layer antireflection (AR) coatings for InP solar cells, such as ZnS, Sb/sub 2/O/sub 3/, SiO/sub 2/ and SiO, deposited either by electron-beam or by resistive evaporation, destroy the stoichiometry of the emitter surface. Consequently, the surface recombination velocity (SRV) at the emitter surface is significantly increased, leading to a reduction in the values of solar cell performance parameters. This can be prevented by growing, after contacting, a thin native oxide layer on the emitter surface. Best results are obtained using a phosphorus-rich chemical oxide grown by chemical oxidation using a newly developed etchant (PNP) based on HNO/sub 3/, o-H/sub 3/PO/sub 4/ and H/sub 2/O/sub 2/. The chemical oxide grown on p/sup +/-InP emitters, using the PNP etchant, passivates the surface and can be used as a first layer AR coating.<<ETX>>

High voltage thermally diffused p/sup +/n solar cells

The possibility of fabricating thermally diffused p/sup +/n InP solar-cells with high open-circuit voltage without sacrificing the short circuit current is discussed. The p/sup +/n InP junctions were formed by Cd and Zn diffusion through a 3-5 nm thick anodic or chemical phosphorus-rich oxide cap layer grown on n:InP:S (with N/sub D/-N/sub A/=3.5*10/sup 16/ and 4.5*10/sup 17/ cm/sup -3/) Czochralski LEC-grown substrates. After thinning the emitter from its initial thickness of 1 to 2.5 mu m down to 0.06-0.15 mu m, the maximum efficiency was found when the emitter was 0.2 to 0.3 mu m thick. Typical AM0, 25 degrees C values of 854-860 mV were achieved for V/sub oc/, J/sub sc/ values were from 25.9 to 29.1 mA/cm/sup 2/ using only the P-rich passivating layer left after the thinning process as an antireflection coating.<<ETX>>

High quality thermally diffused p/sup +/-n InP structures

Cd diffusion and Zn diffusion into n-InP:S (N/sub D/ =3.5*10/sup 16/ and 4.5*10/sup 17/ cm/sup -3/) were performed by a closed ampoule technique at diffusion temperatures from 500 to 600 degrees C by using either high-purity Cd and Zn or Cd/sub 3/P/sub 2/ and Zn/sub 3/P/sub 2/. The Czochralski LEC grown substrates with etch pit densities (EPDs) from 3*10/sup 4/ to 7*10/sup 4/ cm/sup -2/ were used. Diffusions were performed through either bare surfaces or using SiO/sub 2/ (50-100 AA thick) and phosphorus-rich anodic and chemical oxides (25-50 AA thick) as cap layers. Specular surfaces have been obtained after Cd diffusion from Cd/sub 3/P/sub 2/ through P-rich oxide cap layers with a very low surface dislocation density which goes through a minimum of 400-800 cm/sup -2/ at the diffusion temperature of 560 degrees C. AM0 250 degrees C V/sub oc/ values as high as 860 mV from solar cells made on these structures are reported.<<ETX>>

Experiment-based projected high efficiency thermally diffused p/sup +/n (Cd,S) InP solar cells for space applications

By drastically reducing the defect densities of p/sup +/n (Cd,S) InP diffused structures the authors have succeeded in fabricating p/sup +/n InP solar cells with measured AM0, 25/spl deg/C V/sub /spl prop// values exceeding 880 mV, without anti-reflection (AR) coating. Experiment-based projected maximum achievable AM0, 25/spl deg/C efficiency of these cells is 21.3%. Preliminary investigation of the performance parameters of p/sup +/n (Cd,S) InP structures and solar cells after irradiation with 10/sup 13/ cm/sup -2/ 3MeV protons indicate higher radiation tolerance of this configuration as compared to n/sup +/p InP configuration due to its better annealing properties.<<ETX>>

Front surface engineering of high efficiency Si solar cells and Ge TPV cells

We demonstrate the effectiveness of using wet chemical techniques for Si and Ge planar surfaces to form nanoporous layers, and grow stable passivating oxide layers on planar and porous surfaces, after the front grid metallization step. Our results show that this passivated chemical oxide layer: (i) can serve as an effective window/first layer AR coating, (ii) is chemically, thermally and UV stable, (iii) can simplify the structure of Si, Ge and III-V based space solar cells, thereby reducing cost, and (iv) has the potential of improving the BOL and especially the EOL efficiency of Si and III-V based space solar cells.

New three-layer antireflection/surface passivating coating for high efficiency III-V compound solar cells

We use a chemically grown, thermally and chemically stable oxide, not only for surface passivation but also as an integral part of a 3-layer antireflection (AR) coating for thermally diffused p/sup +/n InP solar cells, thus solving the dual problem of surface passivation and surface reflection reduction on these cells. A phosphorus-rich interfacial oxide, In(PO/sub 3/)/sub 3/, is grown at the surface of the p/sup +/ emitter using an etchant based on HNO/sub 3/, o-H/sub 3/PO/sub 4/ and H/sub 2/O/sub 2/. This oxide has the unique properties of passivating the surface as well as serving as a fairly efficient antireflective layer. We show that it is possible to design a three-layer AR coating for a thermally diffused p/sup +/n InP solar cell using the In(PO/sub 3/)/sub 3/-grown oxide as the first layer and either ZnS, MgF/sub 2/ or a new combination MgF/sub 2/ as the second and third layers respectively, so as to yield an overall theoretical reflectance of less than 2%.

Electrolyte for EC-V profiling of InP and GaAs based heterostructures

Electrochemical C-V (EC-V) profiling is the most often used and convenient method for accurate majority carrier concentration depth profiling of semiconductors. None of the previously developed electrolytes recommended for EC-V profiling of InP and GaAs based structures works with all the materials encountered. In our experience, the most common problems encountered when using these electrolytes are: (i) a poor electrolyte/semiconductor Mott-Schottky barrier, (ii) preferential dissolution at the defect areas, (iii) high chemical etch rates, (iv) formation of insoluble products on the surface, (v) large parasitic capacitance components, (vi) a rough bottom of the etch crater, (vii) rounding at the crater rim and (viii) electrolyte seeping under the edge of the sealing rim. Although, according to us, FAP is the best electrolyte for accurate profiling of InP structures, it does not work well with other III-V compounds. To overcome this, recently, we have developed a new electrolyte, which we call UNIEL (UNIversal ELectrolyte). However, as with the FAP electrolyte, the presence of HF makes the UNIEL incompatible with the electrochemical cell of Polaron EC-V profilers manufactured by BIG-RAD. By slightly modifying the electrochemical cell configuration we are able to use both the FAP and UNIEL electrolytes, without destroying the calomel electrode. Recently, we have, nevertheless, experimented with variations of the UNIEL with no HF content for EC-V profiling of structures based on InP and GaAs. Presently available results are presented here.

Closed-ampoule diffusion of sulfur into Cd-doped InP substrates - Dependence of S profiles on diffusion temperature and time

In order to optimize the fabrication of n+–p InP solar cells made by closed‐ampoule diffusion of sulfur into p‐InP:Cd substrates, we have investigated the influence of diffusion conditions on sulfur diffusion profiles. We show that S diffusion in InP is dominated by the P vacancy mechanism and is not characterized by a complementary error function as expected for an infinite source diffusion. The S diffusion mechanism in p‐InP is qualitatively explained by examining the depth profiles of S, P, and In in the emitter layer and by taking into account the presence and composition of different compounds found to form in the In–P–S–O–Cd system as a result of diffusion.

EC-V profiling of InP

The electrochemical current-voltage profiling of p, n, p/sup +/, and n/sup +/ liquid encapsulated Czochralski (LEC) or VGE grown InP substrates. thermally diffused n/sup +/p and p/sup +/n, and epitaxially grown n/sup +/p InP structures using an electrolyte called FAP is described. It found that the FAP electrolyte is inherently superior to previously reported electrolytes (0.5 M HCl and the Pear etch) for performing accurate EC-V profiling of InP at current densities of up to 0.3 mA/cm/sup 2/.<<ETX>>