Charge Accumulation Layer Research Articles

InAs-based heterostructure barrier varactor diodes with In0.3Al0.7As0.4Sb0.6 as the barrier material

InAs-based heterostructure barrier varactor (HBV) diodes with In0.3Al0.7As0.4Sb0.6 as the barrier material are demonstrated. Current–voltage and capacitance–voltage characteristics, as well as S-parameters, of HBV diodes with varying barrier thicknesses are examined. Maximum capacitance values and maximum-to-minimum capacitance ratios greater than those predicted by traditional HBV models were measured. The HBVs’ unconventional behavior in terms of charge accumulation layers adjacent to the wide bandgap barrier is discussed.

Dependence of electric field domain relocation dynamics on contact conductivity in semiconductor superlattices

Numerical simulation results are presented for a discrete drift-diffusion rate equation model that describes electronic transport due to sequential tunneling between adjacent quantum wells in weakly coupled semiconductor superlattices. We study the dependence on contact conductivity $\ensuremath{\sigma}$ of current-voltage characteristics and transient current response to abrupt steps in applied voltage. For intermediate values of $\ensuremath{\sigma}$, three qualitatively distinct transient responses---each associated with a different mechanism for the relocation of a static charge accumulation layer---are observed for different values of voltage step ${V}_{\mathit{\text{step}}}$; these involve, respectively, (1) the motion of a single charge accumulation layer, (2) the motion of an injected charge dipole, and (3) the motion of an injected monopole. A critical value of $\ensuremath{\sigma}$ is identified above which the injected dipole mechanism is not observed for any value of ${V}_{\mathit{\text{step}}}$. Furthermore, at very low $\ensuremath{\sigma}$, we find a reversed static field configuration, i.e., with the high-field domain adjacent to the emitter contact.

Spin Dynamics in Narrow-Gap Semiconductor Epitaxial Layers

We report on investigation of the spin dynamics in InAs and InSb films grown on GaAs at a temperature range from 77 K to 290 K. For both materials, the large lattice mismatch with the GaAs substrate results in the formation of an interface accumulation layer with a large defect concentration, which strongly affects the spin relaxation in these areas. Moreover, the native surface defect in the InAs films resulted in an additional charge accumulation layer with high conductivity, but very short spin lifetime. In contrast, in InSb layers, the surface states introduce a depletion region. We have correlated the spin relaxation with a multi-layer analysis of the transport properties, and find that in a 1 μm thick InAs film, approximately 70% of the total current flows through the interface and surface accumulation layers, which have sub-picosecond lifetimes, whereas in InSb films of the same thickness, the semiconducting layer carries more than 90% of the total current, and the spin lifetime in the accumulation layer is only slightly less than that of the central semiconducting layer. We suggest that InSb could be a more attractive candidate for spintronic applications than InAs.

Spin lifetime in high quality InSb epitaxial layers grown on GaAs

The spin relaxation in undoped InSb films grown on GaAs has been investigated in the temperature range from 77to290K. Two distinct lifetime values have been extracted, 1 and 2.5ps, dependent on film thickness. Comparison of this data with a multilayer transport analysis of the films suggests that the longer time (∼2.5ps at 290K) is associated with the central intrinsic region of the film, while the shorter time (∼1ps) is related to the highly dislocated accumulation region at the film-substrate interface. Whereas previous work on InAs films grown on GaAs showed that the native surface defect resulted in an additional charge accumulation layer with high conductivity but very short spin lifetime, in InSb layers the surface states introduce a depletion region. We infer that InSb could be a more attractive candidate for spintronic applications than InAs.

Charge accumulation layer in Cs,Ba/n-GaN(0001) ultrathin interfaces: Electronic and photoemission properties

Experimental studies and theoretical calculations of the photoemission from Cs/n-GaN(0001) and Ba/n-GaN(0001) ultrathin interfaces were carried out. The electronic properties of the interfaces were studied in situ using threshold photoemission spectroscopy under vacuum at a residual pressure of P ∼ 5 × 10−11 Torr. A new effect was revealed, namely, photoemission with a high quantum yield under excitation with light in the transparency region of GaN. It was shown that adsorption of Cs or Ba on n-GaN brings about the formation of a quasi-two-dimensional electron channel, i.e., a charge accumulation layer directly near the surface. The dependences of the photoemission spectra and work function on the thickness of Cs and Ba coatings were investigated. It was established that adsorption of Cs and Ba leads to a sharp decrease in the work function by ∼1.45 and ∼1.95 eV, respectively. The photoemission spectra were calculated, and parameters of the accumulation layer, such as the energy position of the layer below the Fermi level for different Cs and Ba coverages, were determined. It was demonstrated that the energy parameters of the accumulation layer on the n-GaN(0001) surface can be controlled by properly varying the Cs or Ba coverage. The layer thickness was found to reach a maximum for a cesium coverage of ∼0.5 monolayer.

Selective wet etching of lattice-matched AlInN–GaN heterostructures

Wet etching of AlInN–GaN epitaxial heterostructures, containing AlInN layers with InN mole fractions close to 0.17 has been studied. One molar aqueous solution of the chelating amine 1,2-diaminoethane (DAE) proved to selectively etch the AlInN layers, without the need for heating above room temperature, or photo-assistance. In experiments with a (0 0 0 1)-oriented AlInN-on-GaN bilayer, the mode of removal of the AlInN layer was predominantly lateral etching, initiated from the sidewalls of pit defects in the AlInN layer. The lateral etch rate was estimated at ∼60 nm/h. The GaN buffer layer surface was roughened concurrently with etching of the AlInN, although the DAE solution has no effect on as-grown GaN (0 0 0 1) surfaces. The roughening of the GaN surface is tentatively attributed to the charge accumulation layer expected at the AlInN–GaN heterointerface. The DAE etchant also proved effective at removing buried AlInN layers from trilayer and more complex multilayer structures, leading to the prospect of epitaxial lift-off processes, and the fabrication of three-dimensional engineered microstructures. These capabilities were demonstrated by the production of suspended microdisk structures from a GaN–AlInN–GaN trilayer, using a combination of dry and wet etching.

Creation of a metallic channel at theSn∕InAs(111)Bsurface studied using synchrotron-radiation photoelectron spectroscopy

The properties of a Sn-induced two-dimensional electron gas at the As-terminated $\mathrm{In}\mathrm{As}(111)B(1\ifmmode\times\else\texttimes\fi{}1)$ surface was studied by synchrotron radiation photoelectron spectroscopy. The two-dimensional electron gas reveals itself via a narrow structure at the Fermi level, visible close to normal emission for tin coverage in the range 0.5 to 2 monolayers. Although this electron gas exhibits properties that in several respects resemble those of intrinsic charge accumulation layers on free InAs surfaces, our observations suggest that the present electron gas is much more linked to the Sn adlayer.

Infrared spectroscopy on the charge accumulation layer in rubrene single crystals

To get access to the intrinsic properties of organic semiconductors, investigations on single crystals are essential. The authors report on far- and mid-infrared spectroscopies of the charge accumulation layer in an organic field-effect transistor fabricated on a rubrene single crystal. By charge modulation spectroscopy in the range between 70 and 4750cm−1, they were able to detect the Drude response of the accumulated charges in the channel. From this they can extract important intrinsic transport parameters such as the mobility, the plasma frequency, the effective mass, and the scattering rate.

Charge accumulation layers and surface states in ultrathin Cs,Ba/n-GaN(0001) interfaces

It is found that ultrathin cesium and barium coatings radically change the electronic properties of the surface and the near-surface region of epitaxial n-GaN(0001) layers. A charge accumulation layer serving as a quasi-two-dimensional electronic channel is first formed by adsorption on the surface of a semiconductor. It is revealed that photoemission from the accumulation layer is excited by visible light from the transparency region of GaN and is characterized by a high quantum yield. It is found that the photoemission thresholds h{nu}{sub s} and h{nu}{sub p} for s-and p-polarized excitation are equal to each other and correspond to the work function. The lowest work function for Cs,Ba/n-GaN interfaces is observed at Cs or Ba coverages close to 0.5 monolayer. Two bands induced by the local interaction of cesium (barium) adatoms with gallium dangling bonds are detected in the electronic spectrum of surface states of Cs,Ba/n-GaN interfaces. An oscillation structure is observed in spectral dependences of the photoyield. This effect is new for photoemission. A model of the effect is proposed. It is found that electronic and photoemission properties of the interfaces correlate with the structural perfectness of the epitaxial n-GaN(0001) layers.

Inversion and accumulation layers at InN surfaces

The conduction band minimum (CBM) at the centre of the Brillouin zone (the Γ -point) of wurtzite InN is extremely low compared with the rest of the conduction band-edge. As a result, the charge neutrality level is located high in the conduction band at the Γ -point. This single property of the band structure of InN is the common cause of the observed high unintentional n-type doping, radiation resistance, interface electron accumulation, and inefficiency of p-type doping. Overcoming the challenges and exploiting the opportunities presented by these effects will be critical for future device applications of InN. Here, calculations of charge accumulation and inversion layer profiles at the surfaces of n- and p-type InN, respectively, are presented and discussed. The nature of the band structure of InN, with its particularly low Γ -point CBM, dictates the existence of positively charged donor-type interface states, leading to electron accumulation at InN interfaces.

Widely tunable type-II interband cascade laser

We discuss an ultrawide, voltage-tunable type-II mid-IR interband cascade laser. Its design has a charge accumulation layers outside of the optically active quantum wells that unclamps the electron-hole concentrations and facilitates above-threshold Stark shifts. Our results demonstrate laser tuning of 120nm (120cm−1).

Simulation of Band Diagram for Chemical-Vapor-Deposition Diamond Surface Conductivity

One-dimensional energy band simulation has been performed in order to understand chemical-vapor-deposition (CVD) diamond surface conductivity. It was found that the presence of shallow-level acceptors in the subsurface region and defect states at the surface causes a steep rise in the valence band toward the Fermi level, which causes accumulation of holes in the valence band in the subsurface and near-surface regions. An artificial negative charge accumulation (NCA) layer is introduced in the simulation to examine the effect of possible negatively charged adsorbates on surface conductivity. By adjusting the thickness of NCA layers, we have reproduced quantitatively both the surface conductivity change and Fermi-level change found in previous experiments [Kono et al.: Diamond Relat. Mater. 14 (2005) 459; Riedel et al.: ibid.13 (2004) 746].

Surface passivation of InAs(001) with thioacetamide

We describe the passivation of InAs(001) surfaces with thioacetamide (CH3CSNH2 or TAM) as an alternative to the standard sulfur passivation using inorganic sulfide (NH4)2Sx. Quantitative comparison using x-ray photoelectron spectroscopy (XPS) demonstrates that TAM passivation dramatically improves the stability against reoxidation in air compared with the inorganic sulfide, with little to no etching during the treatment. We find that TAM passivation preserves the intrinsic surface charge accumulation layer, as directly confirmed with laser-induced photoemission. Overall, TAM appears to provide superior passivation for electronic device and sensing applications.

Charge accumulation layer on the n-GaN (0001) surface with ultrathin Ba coatings

The adsorption of Ba on the n-type GaN(0001) surface is studied. It is found that submonolayer Ba coatings induce cardinal changes in the electronic properties of the surface with the formation of a charge accumulation layer in the region of the near-surface band bending. The excitation of the Ba/n-GaN system by light from the region of GaN transparency results in photoemission. The lowest value of the work function corresponds to ∼1.90 eV at a Ba coverage of ∼0.4 ML. Two surface bands induced by Ba adsorption are found in the surface photoemission spectra.

Anisotropy of electron structure at InAs(1 1 1) surfaces by laser pump-and-probe photoemission spectroscopy

The electronic structure and the electron dynamics of the clean InAs(1 1 1)A 2 × 2 and the InAs(1 1 1)B 1 × 1 surfaces have been studied by laser pump-and-probe photoemission spectroscopy. Normally unpopulated electron states above the valence band maximum (VBM) are filled on the InAs(1 1 1)A surface due to the conduction band pinning above the Fermi level ( E F). Accompanied by the downward band banding alignment, a charge accumulation layer is confined to the surface region creating a two dimensional electron gas (2DEG). The decay of the photoexcited carriers above the conduction band minimum (CBM) is originated by bulk states affected by the presence of the surface. No occupied states were found on the InAs(1 1 1)B 1 × 1 surface. This fact is suggested to be due to the surface stabilisation by the charge removal from the surface into the bulk. The weak photoemission intensity above the VBM on the (1 1 1)B surface is attributed to electron states trapped by surface defects. The fast decay of the photoexcited electron states on the (1 1 1)A and the (1 1 1)B surfaces was found to be τ 1 1 1 A ⩽ 5 ps and τ 1 1 1 B ⩽ 4 ps, respectively. We suggest the diffusion of the hot electrons into the bulk is the decay mechanism.

Electron accumulation layer at the Cs-covered GaN(0001) n-type surface

We report on the observation of photoemission from the Cs∕GaN(0001) n-type interface by excitation of visible light in the transparency region of GaN. Under Cs adsorption, sharp decrease in photoemission threshold up to 1.3eV at 0.5 monolayer of Cs is found and shown to be due to formation of a charge accumulation layer in the near-interface region. An interesting phenomenon is revealed, namely, the appearance of an oscillation structure in spectra of photoyield. A model conception taking into account both the formation of charge accumulation layer and occurrence of multiple-beam interference in parallel-sided GaN epilayer is suggested.

Chemical and electronic properties of sulfur-passivated InAs surfaces

Treatment with ammonium sulfide ((NH 4) 2S x ) solutions is used to produce model passivated InAs(0 0 1) surfaces with well-defined chemical and electronic properties. The passivation effectively removes oxides and contaminants, with minimal surface etching, and creates a covalently bonded sulfur layer with good short-term stability in ambient air and a variety of aqueous solutions, as characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and Hall measurements. The sulfur passivation also preserves the surface charge accumulation layer, increasing the associated downward band bending.

Pulsed Electroluminescence in a Para-Hexaphenyl Based Heterostructure Device

We present the performance of an organic blue-light emitting device under quasi-stationary pulsed conditions. The transient current and light emission shows a fast initial response that is followed by a continuous increase of the light output. This behaviour, which is observed on a millisecond time-scale is attributed to the formation of a charge accumulation layer at the organic layer/cathode interface, together with field enhanced electron injection. Furthermore, it is shown that reverse biasing the device prior to the actual driving pulse increases the external quantum efficiency, which results into an enhancement of light emission up to 25% by virtue of depleting space charge carriers and charge-filled trap sites.

Charge accumulation layer induced by potassium adsorption on hydrogen terminated Si(111)(1×1)

By ultraviolet photoemission spectroscopy and high-resolution electron energy loss spectroscopy we have detected the formation of a charge accumulation layer at the surface of a hydrogenated Si(111)(1×1) single crystal at very low potassium coverages. By UPS we have followed the evolution of the band edges relative to the Fermi level, leading to a downwards band-bending (BB) around 340 meV, at about 0.05 of the K saturation coverage. This means that the conduction band bottom is below the Fermi level, producing the accumulation of charge carriers just below the surface. At increasing coverages the BB retains its downward trend, reducing its value to 100 meV at K saturation. The presence of an accumulation layer, consisting of a quasi two-dimensional electron gas, is unambiguously confirmed by the anomalous behaviour of the quasi-elastic peak in the HREEL spectra. Its width strongly increases at the lowest coverage and remains quite large up to the coverage at which UPS shows that the Fermi level leaves the CB minimum.

Raman scattering as a probe of the superconducting proximity effect

Temperature-dependent Raman spectroscopy is used to investigate the effect of superconductivity on the near-surface electronic structure of a semiconductor in good electrical contact with a superconductor. The light scattering is performed through a high-quality thin (60–100A) film of Nb, grown directly ontoin-situ Ar+-etched (100)-n+InAs. Below Tc, the LO mode, associated with the surface charge accumulation layer in the InAs, is enhanced by ∼40% in comparison with the nearby L_bulk phonon mode. This change, reversible upon temperature cycling, is observed only when the Nb is in good electrical contact with the InAs. Preliminary results show a similar effect on NbN/InAs. Our results constitute the first optical detection of the superconducting proximity effect.