Photoconductivity study of semiconductors using the surface acoustic wave convolver

Photoconductivity study of a semiconductor using the SAW convolver is presented. The semiconductor is placed either on or a small distance above the SAW delay line. A fast rise time pulse of light is applied to the semiconductor surface through the piezoelectric substrate, and the resulting change in the semiconductor conductance is observed by the relaxation of the SAW propagation loss. This technique has the important advantage that no ohmic contact to the semiconductor is needed.     

Investigations of interface-state density in SiMOS structures

Admittance measurements of n-type MOS capacitors indicate a large density of surface states near the conduction band and a small hillock or peak of surface state density near the midgap of the silicon energy band gap. The conductance peak related to the surface states at midgap was found to be sensitive to temperature changes of the system. These surface states we associate with the dangling bonds of Si at the interface. The time constant and hence the capture cross-section of these surface states were found to be different than those at other positions in the Si band gap.     

Theory of transient emission current in MOS devices and the direct determination interface trap parameters

The physics is discussed of the emission of electrons from interface states in metal-insulator-semiconductor (MIS) systems, under isothermal, non-steady-state conditions. Generalized equations are then derived which permit the determination of the non-steady-state, emission current vs time characteristics for MOS systems containing an arbitrary distribution of surface states; the special case of a discrete surface state is also studied. More important, however, by appropriate plotting of the data, it is shown how to directly extract from the experimental data the energy distribution and the capture cross section of the interface traps in the upper-half of the band gap in the case of n-type semiconductors, and in the lower-half of the band gap in the case of p-type semiconductors.     

Characterization of surface states in HCl-grown oxides using MOS transient currents

A fast transient current (TC) technique has been developed for the characterization of majority carrier charge emission from surface states using MOS capacitors excited by a voltage step-function. This technique, with appropriate choice of initial and final biasing conditions, allows a rapid determination of the density of surface states (N_ss) and their capture cross section values (σ_n) in preselected regions of band gap using suitable temperature ambients. A low temperature (113°K) was used for regions close to the bottom of conduction band and room temperature and moderately low temperatures were used for the mid-gap region. Results of transient current measurements were compared with those obtained from thermally stimulated current and low frequency C-V measurements. The MOS devices were fabricated using [100] oriented n-type (6–8 Ω-cm) silicon on n⁺ substrates with HCl added to the oxidizing ambient. The detectability limit of the TC technique has been found to be approximately 1 × 10¹⁰ cm^?2 eV^?1 for the device area used.     

Photo-thermal probing of Si-SiO2 surface centers—II: Experiment

The results of photo-thermal probing measurements are presented and interpreted to characterize the Si-SiO₂ surface center photoresponse and to provide information relevant to the evaluation of existing surface state models. Data is first presented to indisputably confirm the facts that surface center photoemission had indeed been observed and that the photoresponse could be isolated from competing relaxation mechanisms. The subsequent presentation is devoted primarily to an examination and analysis of photovoltage vs time data characterizing the surface center response. From the analysis it is concluded that two distinct types of surface centers are quasi-continuously distributed in energy over the central portion of the Si band gap, with both types of states acting as if they were positioned right at the Si-SiO₂ interface. The feature distinguishing the two types of states, referred to as A-states and B-states, is a widely different photocapture cross section at any given band gap energy, measured photocapture cross sections being on the order of 10^?19 cm² and 10^?20 cm² for A- and B-states, respectively. B-states, which exhibit the longer photorelaxation time constant, dominate the response in the upper portion of the band gap, while A-states dominate the response below approximately E_v + 0.3 eV. Finally, the photocapture cross section of each type of state was found to increase systematically toward the band edges due to a Lucovsky-type energy dependence.     

A study of the interface states in MIS-structures with thin SiO2 and SiOxNy layers using deep level transient spectroscopy

Deep level transient spectroscopy (DLTS) and quasistatic CV measurements were used for the study of the interface states of thin SiO₂ and SiO_xN_y layers of 6–9 nm thickness, grown by rapid thermal processing in O₂ or N₂O ambient. DLTS was applied either in the saturating pulse or in the small pulse (energy resolved) mode. From an Arrhenius evaluation of the peaks obtained by temperature-scan measurements with small pulse excitation we derived the distribution of the capture cross section σ_n in the upper half of the gap, which exhibits a drastic decrease towards the conduction band edge.     

Low-temperature hysteresis effects in metal-oxide-silicon capacitors caused by surface-state trapping

At low temperatures, charge exchange in all surface states except those close to the band edges can occur only by capture of free carriers because emission rates become very slow. If means are provided to supply minority carriers (either from an extended inversion layer or in a gate-controlled diode), pronounced charge-trapping effects can be observed. A ledge in the C-V characteristic is identified as being due to the charging of almost all surface states within the forbidden gap at a surface potential dependent on surface-state density, capture cross section and voltage sweep rate. Capture cross sections at low temperatures can be estimated from the onset of the ledge. When the C-V curves are traced from accumulation to inversion the capacitance drops below the equilibrium minimum value into depletion and increases rapidly when inversion is reached. This "hook" is caused by a barrier against minority carrier flow at the boundary of the MOS capacitor. The barrier disappears when sufficient voltage is applied to charge the surface states in the boundary region.     

Electrical surface properties of semi-insulating and ion implanted GaAs revealed by thermo-optical acousto-electric voltage method

A thermo-optical acousto-electric voltage method is used for the determination of electrical surface properties of GaAs materials. Measurements conducted on semi-insulating substrates, some with an implanted layer, allow determination of surface conduction type, impurity activation, and photoconductivity behavior. The spectroscopic nature of the technique allows detection and characterization of energy levels in the semiconductor band gap. For the samples tested a 0.87 eV level attributed to chromium and a broad distribution of acceptor type surface states about 1.0–1.05 eV from valence band are reported. The main advantages of this measurement method are its effectiveness for the study of semiconductors over a wide range of resistivities and its nondestructive character.     

Very slow charge trapping and release in ion implanted GaAs[MESFETs]

Unusually slow trapping and release of electrons has been observed in ion implanted GaAs with room temperature time constants of the order of 1000 s. Trap filling experiments showed that the capture cross section was temperature dependent with capture being more efficient at lower temperatures. The measured capture cross sections were used to refine the analysis of the emission data to demonstrate the existence of two electron emission processes with activation energies of 0.56 eV and 0.013 eV. The effects appear not to be artefacts due to surface or interface states at the edge of the test structures but are due to states within the bulk of the semiconductor within the implanted region     

一维固-固结构声子晶体中SH波的声场分布

利用转移矩阵法推导出SH波在一维固-固结构声子晶体中声场的分布公式,利用这个公式研究了一维固-固结构声子晶体中声场的分布特征。禁带中声场随着SH波在一维固-固结构声子晶体内部传播深度的增加而迅速衰减。导带中声场不随SH波的传播深度的增加而衰减。这些特征的获得深化了对SH波在一维固-固结构声子晶体的禁带和导带的形成的认识。     

On occupation functions of donor- and acceptor-like interface states in metal-insulator-semiconductor tunnel structures

This paper presents a theoretical study on the occupation functions of interface states in MIS tunnel structures. Based on Shockley-Read-Hall (SRH) statistics and considering carriers tunneling between the metal and interface states, occupation functions of both donor- and acceptor-like interface states at arbitrary energy level within the semiconductor band gap are derived and analyzed. It reveals that, for the same energy level, the occupation functions of donor-like and acceptor-like interface states are remarkably different in magnitude. The deviation in occupation functions of these two types of interface states is a nonlinear function of the ratio of capture cross-section of the charge states to that of the neutral states (C_C/C_N) and the semiconductor surface conditions. If the insulating layer (SiO₂) is relatively thick (> 50 Å) or thin (< 10 Å), interface states are in equilibrium with the semiconductor or the metal, respectively. Under the circumstances, the occupation functions of the two types of interface states are no longer distinguishable. Alternatively, they can be approximated by the well-known Fermi-Dirac distribution function.

In this paper, quantitative influences of key parameters of MIS tunnel structures such as insulating layer thickness, electron and hole density at the semiconductor surface, capture cross-sections for charged and neutral states, etc., on interface states occupation function are discussed. For Gaussian-distributed donor- and acceptor-like interface states, the quantitative roles of interface states in charge storage and current-assisting effects are also demonstrated.     

A.C. field effect study on PbTe films grown by hot wall epitaxy technique

The field effect mobility on the free surface of PbTe films grown by hot wall epitaxy (HWE) technique ona KCl substrate was studied as a function of frequency in the range 40–200 kHz at different temperatures from 98–156 K. Using Garrett's theory of the frequency dependence of field-effect mobility, the relaxation times of surface states were found to vary from 1.84 to 0.96 μsec in the above temperature range. From Rupprecht's relation on temperature dependence of relaxation times, the energy level of surface states was found to be 0.02 eV below the conduction band with an effective capture cross section of the order 10^?19 cm².     

Low-temperature photocapacity measurement in MOS structure

In this paper a method for the investigation of the fast Si---SiO₂ interface states by spectral distribution of photocapacity of the MOS system at low temperatures is presented. The method is based on the photostimulated emission of carriers from the surface into the bulk. Associated change of MOS capacity due to summing up of the charge at surface makes this method more sensitive than other optical methods. The typical photocapacity transient is discussed. For illustration, the spectral distribution of the initial rate of total capacity change is shown for the n-type MOS-C.

It is suggested that the observed effects are due to photostimulated transitions from single-level states or a narrow band lying at about 0·4 eV below the conduction band edge. The lack of effects which can be attributed to the transitions from continuously distributed surface states suggests that previously observed surface states could be trapping centres located in the oxide at or near the Si---SiO₂ interface.     

Charge-transfer noise theory for surface-channel charge-coupled devices

The detailed expression of charge-transfer noise for surface-channel CCD's is derived from the surface generation-recombination rate equation. Not only the charge fluctuation due to occupied interface states but also that due to unoccupied interface states are taken into account. The derived expression can explicitly predict the spectrum intensity dependences on the signal charge concentration and the clock frequency for various amounts of the fat-zero charge. The comparison between theory and experimental results shows good agreement over a wide range of signal charge concentration and clock frequency. It is shown that charge-transfer noise depends heavily on the ratio of the signal charge concentration to the clock frequency and the capture cross section in the absence of fat-zero charge. For the small ratio, the charge fluctuation is ruled by the transfer-process noise which is determined from the amount of unoccupied interface states. For the very large ratio, the storage-process noise which is determined from the interface state distribution over the band gap rules the charge fluctuation. Fat-zero charge hardly reduces the transfer-process noise for the small ratio. For the large ratio, the fat-zero charge enhances the total transfer noise.     

AlGaAs:Si中与DX中心有关的光生电子陷阱特性研究(英文)

A new electron trap state SD was found by DLTS measurement under light illumination in Si doped A.lxGa1-xAs. This new trap energy level ESD is shallower than the DX center energy in the gap and the concentration of SD is comparable to that of DX centers. The emission activation energy Ec=0.20±0.05eV and capture activation energy Ec= 0.17±0.05eV. The SD DLTS peak has never been detected previously because under dark and thermal equilibrium condition most of the electrons occupy the deeper DX states and most of SD states are empty. However, when the sample is illuminated by light, electrons are excited to the conduction band and then re-captured by SD since the deeper DX states have a slower electron capture rate, thus a new DLTS peak corresponding to SD appears. Constant temperature capacitance transient C-t and transient C-V measurements were also used to further confirm the existence of SD states.     

Carrier generation, recombination, trapping, and transport in semiconductors with position-dependent composition

The spatial variation of the chemical composition of a semiconductor modifies the ideal one-electron energy band model as well as the Shockley equations for carrier recombination and transport in two important ways. The random component of the spatial variation introduces localized states in the energy gap by perturbing the band states.The nonrandom component gives rise to the position dependences of the conduction and valence band edges or the electron affinity and the energy gap. This paper gives the modifications of the Shockley equations from these two effects as well as an example of the steady-state recombination rate from distributed gap states in junction solar cells.     

Theory of nonequilibrium properties of MIS capacitors including charge exchange of interface states with both bands

An improved model of the nonequilibrium behaviour of MIS varactors is presented taking into account the charge exchange of interface states with both bands. C–V curves were calculated for donor and acceptor type interface states of typical distribution in energy. By means of the proposed model the bulk minority carrier generation time constant can be measured and related with semiconductor bulk properties. This allows one to distinguish between interface states acting with the conduction band and those interacting with the valence band. A possible experimental proof of the presented model as well as its limitations are discussed in detail.     

The effects of oxide traps on the MOS capacitance

The trapping of electrons and holes at a semiconductor surface by traps located in the oxide adjacent to the semiconductor has been considered. It is shown that the effective capture cross section of an oxide trap viewed by a carrier at the semiconductor surface is reduced by a factor which increases exponentially with the distance the trap is located from the interface. A pseudo-Fermi function in this position variable is developed which gives the probability that a trap will be filled (or emptied) in a measurement time, Tm. The trapping kinetics developed in the first part of the paper are applied to yield the full frequency and bias dependence of an MOS capacitor for an arbitrary spatial and energy trap distribution. Specific examples are given and the problem of voltage hysteresis is dealt with quantitatively. The conclusion is that very little information about the energy distribution and capture cross sections of the oxide traps is obtained from the analysis of MOS-capacitance curves.     

钒受主能级在n型4H-SiC中的深能级瞬态傅里叶谱和光致发光

借助深能级瞬态傅里叶谱研究了钒离子注入在SiC中引入的深能级陷阱.掺人的钒在4H-SiC中形成两个深受主能级,分别位于导带下0.81和1.02eVt处,其电子俘获截面分别为7.0 × 10-16和6.0×10-16cm2.对钒离子注入4H-SiC样品进行低温光致发光测量,同样发现两个电子陷阱,分别位于导带下0.80和1.6eV处.结果表明,在n型4H-SiC掺入杂质钒可以同时形成两个深的钒受主能级,分别位于导带下0.8±0.01和1.1±0.08eV处.     

Effects of surface states on relaxation phenomena in MOS capacitors

A numerical simulation of a silicon MOS capacitor pulsed from equilibrium into deep inversion has been performed with a view to accurately investigating the effects associated with surface states. A continuum of acceptor surface states has been assumed across the energy gap but, of course, the conclusions may be extended to the case of donor-like states. The results of the calculations show that soon after application of the voltage step a considerable portion of the trapped electrons is released from the upper half energy gap into the conduction band giving rise to a spike in the external current. The emission of these electrons is so rapid that it cannot be detected by the usual slow response current meter, nor is it able to induce any relevant variation into the depletion layer width and, consequently, into the high-frequency capacitance. At the same time, electrons trapped in the surface states disappear also through recombinations with holes generated in the bulk. During a time interval, which turns out to be small in comparison to the duration of the whole transient, every hole generated recombines at the surface giving rise to a small delay in the formation of the inversion layer. After that this latter begins to build up and surface states behave as if they were in a quasi-equilibrium condition with the valence band.