Anja Dobrich

Interplay of amorphous silicon disorder and hydrogen content with interface defects in amorphous/crystalline silicon heterojunctions

We analyze the dependence of the interface defect density Dit in amorphous/crystalline silicon (a-Si:H/c-Si) heterojunctions on the microscopic properties of ultrathin (10 nm) undoped a-Si:H passivation layers. It is shown that the hydrogen bonding and network disorder, probed by infrared- and photoelectron spectroscopy, govern the initial Dit and its behavior upon a short thermal treatment at 200 °C. While the initial Dit is determined by the local and nonequilibrated interface structure, the annealed Dit is defined by the bulk a-Si:H network strain. Thus it appears that the equilibrated a-Si:H/c-Si interface does not possess unique electronic properties but is governed by the a-Si:H bulk defects.

Si(100) surfaces in a hydrogen-based process ambient

We studied the atomic surface properties of Si(100) during preparation in a (metal-organic) vapor phase epitaxy (VPE) reactor and the impact of the hydrogen ambient. Absorption lines in Fourier-transform infrared spectra were identified as stretch modes of coupled Si–H monohydrides, in agreement with Si-dimers observed by scanning tunneling microscopy. The polarization dependence of the antisymmetric stretch mode distinguished different dimer orientations and verified a clear preference for one of the (2×1)/(1×2) surface reconstruction domains. Tip-induced H-desorption proved the complete saturation of dangling bonds after VPE-preparation. In situ reflectance anisotropy spectroscopy showed the absence of Si–H bonds at elevated annealing temperature.

Quantitative investigation of hydrogen bonds on Si(100) surfaces prepared by vapor phase epitaxy

The authors investigated Si(100) surfaces prepared by vapor phase epitaxy (VPE) using Fourier transform infrared spectroscopy (FTIR) in an attenuated total reflection configuration and low energy electron diffraction (LEED). They detected the symmetric and antisymmetric stretch modes of the H–Si–Si–H monohydrides using FTIR in agreement with the associated (2×1)/(1×2) LEED patterns. Polarized FTIR measurements verified the surface character of the observed hydrogen bonds. Exchanging the process gas in our VPE reactor to argon at an intermediate temperature of around 700 °C showed the impact of the hydrogen ambient during the cooling phase at the end of the process. The authors were able to obtain a strong preference of one of the two possible surface domains by variation of the cooling procedure and quantified the domain ratio by comparison of the absorption due to the antisymmetric modes in polarized spectra parallel and perpendicular to the plane of incidence.

Controlling the polarity of metalorganic vapor phase epitaxy-grown GaP on Si(111) for subsequent III-V nanowire growth

Nanowire growth on heteroepitaxial GaP/Si(111) by metalorganic vapor phase epitaxy requires the [-1-1-1] face, i.e., GaP(111) material with B-type polarity. Low-energy electron diffraction (LEED) allows us to identify the polarity of GaP grown on Si(111), since (2×2) and (1×1) surface reconstructions are associated with GaP(111)A and GaP(111)B, respectively. In dependence on the pre-growth treatment of the Si(111) substrates, we were able to control the polarity of the GaP buffers. GaP films grown on the H-terminated Si(111) surface exhibited A-type polarity, while GaP grown on Si surfaces terminated with arsenic exhibited a (1×1) LEED pattern, indicating B-type polarity. We obtained vertical GaAs nanowire growth on heteroepitaxial GaP with (1×1) surface reconstruction only, in agreement with growth experiments on homoepitaxially grown GaP(111).

An experimental-theoretical atomic-scale study - in situ analysis of III–V on Si(100) growth for hybrid solar cells

We consider GaP/Si(100) as quasi-substrate for III-V-on-silicon growth targeting solar energy exploration in dual junction devices for both photovoltaics as well as photoelectrochemical tandem diodes with optimum bandgaps. We prepare Si(100) surfaces with majority domains of either type, grow thin GaP layers free of anti-phase disorder, find that abrupt Si-P interfaces are favored over abrupt Si-Ga interfaces and, finally, observe an RAS signal attributed to N incorporation in GaPN/Si(100). Combining in situ reflection anisotropy spectroscopy during metalorganic vapor phase epitaxy with UHV-based surface techniques and ab initio DFT calculations, we aim to understand the interface formation at the atomic scale.

Minority carrier lifetime analysis of bulk and interfaces on MOVPE grown III-V Low bandgap solar cell materials

We report on time- and spatially resolved photoluminescence measurements using InP/GaInAs/InP double hetero structures (DH-structures) to determine the crucial minority carrier lifetime for the Ga0.47In0.53As to InP interface which is basic for optoelectronic and photovoltaic applications. Different preparation routes, such as group III- and group V-rich variations are presented with respect to their lifetimes, interface recombination velocity and lateral interface homogeneity. From a series of different thick DH-structures at a fixed excitation density for chosen interface preparation routes the bulk lifetime and interface recombination velocity are extracted. Overall low interface recombination velocities occur, while one group III-rich preparation route results in comparable lifetimes at all excitation densities to the group V-rich preparation but with a much higher lateral homogeneity.

MOVPE-preparation of Si(111) surfaces for III–V nanowire growth

We studied the preparation of the clean Si(111) surface in H2 ambient with in situ reflection anisotropy spectroscopy and UHV-based surface science tools after contamination-free transfer. X-ray photoelectron spectroscopy confirmed complete oxide removal after high-temperature annealing. In situ RAS enabled observation of the oxide removal in dependence of process temperature. Monohydride termination was verified by Fourier transform infrared spectroscopy which agrees with a (1×1) surface reconstruction we observed by scanning tunneling microscopy and low energy electron diffraction. By atomic force microscopy analysis of the morphology, we found that wet-chemical pretreatment has an impact on the different silicon surfaces we have prepared, including homoepitaxy and termination of silicon with arsenic.

Preparation of single-domain Si(100) surfaces with in situ control in CVD ambient

III-V films grown heteroepitaxially on Si(100) substrates by metal-organic chemical vapor deposition (MOCVD) are desired for the combination of optoelectronics with microelectronic devices. Difficulties regarding device quality are related to the formation of the crucial III-V/Si(100) interface, where single-layer steps on the substrate surface induce antiphase disorder in the epitaxial film. In principle, double-layer steps on the Si(100) substrate prevent the occurrence of antiphase disorder. While the preparation of silicon surfaces is well-established in UHV, preparation in H2 ambient differs considerably. Considered energetically least favorable on both the clean and the monohydride-terminated Si(100) surface, single domain surfaces with double layer steps in the unusual DA configuration were recently prepared in MOCVD ambient. The DA step formation on Si(100) with 2° offcut in CVD ambient is suggested to originate in vacancy generation and diffusion on the terraces accompanied by preferential annihilation at the step edges. Here, we investigate Si removal and vacancy formation on Si(100) substrates with large terraces under CVD preparation conditions. With in situ reflection anisotropy spectroscopy (RAS), we directly observe the domain formation in dependence of the preparation route. Oscillations in transient RA measurements indicate layer by layer Si removal during annealing in hydrogen. Based on scanning tunneling microscopy results, we conclude that vacancy island formation and anisotropic expansion preferentially in parallel to the dimer rows of the terraces explains the layer-by-layer Si removal process.

Double-layer stepped Si(100) for III–V-on-silicon integration

We demonstrate the formation of anomalous atomic double-layer surface steps on 2° misoriented Si(100) in hydrogen process ambient. Employing a contamination-free sample transfer, low energy electron diffraction and atomic resolution scanning tunneling microscopy reveal dimer rows running parallel to the step edges, i.e. DA type steps, thought to be energetically unfavorable. Based on the interaction of the Si(100) surface with the hydrogen ambient, we propose a model whereby step formation results from interplay of surface vacancy generation, diffusion, and annihilation at the step edges. Reflection anisotropy spectroscopy enables in situ observation of Si(100) surface formation and confirms our model.

In situ control of Si(100) and Ge(100) surface preparation for the heteroepitaxy of III-V solar cell architectures

Si(100) and Ge(100) substrates essential for subsequent III-V integration were studied in the hydrogen ambient of a metalorganic vapor phase epitaxy reactor. Reflectance anisotropy spectroscopy (RAS) enabled us to distinguish characteristic configurations of vicinal Si(100) in situ: covered with oxide, cleaned by thermal removing in H2, and terminated with monohydrides when cooling in H2 ambient. RAS measurements during cooling in H2 ambient after the oxide removal process revealed a transition from the clean to the monohydride terminated Si(100) surface dependent on process temperature. For vicinal Ge(100) we observed a characteristic RA spectrum after annealing and cooling in H2 ambient. According to results from X-ray photo electron spectroscopy and Fourier-transform infrared spectroscopy the spectrum corresponds to the monohydride terminated Ge(100) surface.