Two-dimensional Electron Gas Concentration Research Articles

Molecular beam epitaxy growth of indium-rich [formula omitted] structures towards high channel conductivity for a high electron mobility transistor using a linearly graded buffer layer

In x Ga 1 − x As In y Al 1 − y As heterostructure with both high electron mobility and high carrier concentration has been fabricated on InP substrate by molecular beam epitaxy. The measured electron mobilities were 11 491 cm 2/V · s at 300 K and 53 316 cm 2/V · s at 77 K for two-dimensional electron gas concentrations of 4.7 × 10 12 and 3.5 × 10 12 cm −2, respectively. The high electron mobility and concentration resulted from the dislocation-free, relatively thick (200 Å), and high indium content (80%) channel layer. The high-quality In x Ga 1 − x As channel layer was successfully grown at a growth temperature of 520°C after introducing a linearly graded In y Al 1 − y As buffer structure grown at a reduced growth temperature of 420°C. The cross-sectional transmission electron microscopy observation revealed that the dislocations generated due to a large lattice mismatch between In y Al 1 − y As and InP substrate were locked up in the middle of the graded buffer layer. We believe that we have achieved to date for this materials system the highest room-temperature conductivity (mobility times carrier concentration) of 5.4 × 10 16/V · s.

Study of photoconductivity in AlxGa1−xAs/GaAs modulation-doped heterostructures

Temperature-dependent Hall effect measurements under different illumination conditions are performed on AlxGa1−xAs/GaAs heterostructures doped with both Si and Sn. For a Sn-doped AlxGa1−xAs/GaAs heterostructure with x=0.35, two distinct DX center levels are observed directly without exposure to light. The two-dimensional electron gas concentration measured under illumination decreases from the value due to persistent photoconductivity to a value less than that measured in the dark when the excitation photon energy is larger than the band gap of the AlGaAs barrier. This decrease occurs only at temperatures below 90 K. This negative photoconductivity is explained by taking into account the partial freeze-out of electrons into the shallow DX centers and the transfer of holes photogenerated in the barrier into the channel region. No such phenomena are observed in Sn-doped heterostructures with x <0.35 or in the Si-doped heterostructures. The reasons for this are also investigated.

Photoreflectance study on the two-dimensional electron gas of InAlAs/InGaAs heterojunction bipolar transistor grown by molecular beam epitaxy

InAlAs/InGaAs heterojunction bipolar transistor with a 300 Å spacer inserted between emitter and base grown by molecular beam epitaxy was characterized by using photoreflectance spectroscopy. The energy features observed above the InGaAs fundamental band gap are attributed to the quantum confined subband transition of two-dimensional electron gas which was confined in the spacer channel. A detailed lineshape fit makes it possible to evaluate the Fermi energy, and hence the two-dimensional electron gas concentration. The behavior of two-dimensional electron gas at temperatures between 10 and 300 K was also characterized and the sheet-density in proportion to temperature was observed. Furthermore, using the temperature dependence of effective mass measured by cyclotron resonance combined with photoreflectance to analyze built-in electric field is also reported.

Mobility enhancement in a highly strained δ-doped In0.9 Ga0.1 P/In0.75 Ga0.25 As/In0.53 Ga0.47 As/InP heterostructure

An In0.9 Ga0.1 P/In0.75 Ga0.53 Ga0.47 As/InP heterostructure grown by low-prssure metal-organic chemical vapor deposition exhibits a mobility as high as 7910 cm2V−1s−1 with a two-dimensional electron gas (2DEG) concentration of 3.35 × 1012 cm−2 at 300 K (99 300 cm2 V−1 s−1, 1.76 × 1012 cm−2 at 77 K). Photoluminescence analysis shows good carrier confinement in this structure.

Room-Temperature Photoreflectance Characterization of InAlAs/InGaAs Heterojunction Bipolar Transistor Structure Including Two-Dimensional Electron Gas

Using contactless photoreflectance at 300 K we have characterized two InAlAs/InGaAs heterojunction bipolar transistor structures grown by molecular beam epitaxy. The spectra from the InAlAs and InGaAs interface can be accounted for on the basis of a triangular potential well which confined two-dimensional electron gas. A detailed lineshape fit makes it possible to evaluate the Fermi energy, and hence the two-dimensional electron gas concentration. Furthermore, other important parameters of the system, such as built-in electric fields and In composition, can be evaluated.

Enhanced Two-Dimensional Electron Gas Concentrations and Mobilities in Multiple δ-DopedGaAs/In0.25Ga0.75As/GaAs Pseudomorphic Heterostructures

Selectively multiple δ- doped GaAs/In0.25Ga0.75As/GaAs pseudomorphic heterostructures grown by low-pressure metalorganic chemical vapor deposition (LP-MOCVD) for improving two-dimensional electron gas (2DEG) concentrations and mobilities were demonstrated. All the multiple δ- doped structures revealed significantly higher 2DEG concentration and mobility than those of single δ- doped and conventional homogeneously doped GaAs/InGaAs structures. The structure with double δ- doped GaAs layers grown symmetrically on both sides of the channel showed extremely high 2DEG concentrations of 6.2(4.1)×1012 cm-2 and mobilities of 4630(19100) cm2/ v· s at 300(77) K. Meanwhile, a peak extrinsic transconductance of 390 mS/mm, maximum effective saturation current density of 880 mA/mm, with broad and high transconductance region at 300 K, were achieved in this symmetrically δ- doped GaAs structure.

Self-consistent simulation of modulation-doped field-effect transistors

Due to the fact that conventional linear charge-control modulation-doped field-effect transistor (MODFET) models assume full depletion in the doping layer, they are not able to model the nonlinear charge-control relation arising from neutralized donor effect near large gate bias. This neutralized donor effect, also called parasitic MESFET effect, saturates two-dimensional electron gas (2DEG) concentration and raises gate-source capacitance to make high frequency performance poor. In this paper, electron behavior of MODFET along the growth direction is numerically modeled through self-consistent calculation to achieve a more exact charge-control relation. Then, 2DEG channel mobility is described by a semi-empirical analytical form and substituted into a current-density equation together with the charge-control relation fitted by least-square polynomial in order to easily solve for drain-source current and gate transconductance. These theoretically calculated results are in good agreement with those experimental results presented in the literature.

Influences of δ -doping time and spacer thickness on the mobility and two-dimensional electron gas concentration in δ -doped GaAs/InGaAs/GaAs pseudomorphic heterostructures

A series of GaAs/In0.25Ga0.75As/GaAs pseudomorphic high electron mobility transistors (HEMTs) was grown by low-pressure metalorganic chemical vapor deposition. The respective influences of δ-doping time and spacer thickness on the two-dimensional electron gas concentrations and the mobilities were studied. The maximum mobilities, μ=5600 and 22 000 cm2/V s at 300 and 77 K, respectively, appeared at a spacer thickness of 100 Å with a δ-doping time of 0.2 min. A δ-doped GaAs/In0.25Ga0.75As/GaAs pseudomorphic HEMT with the above parameters was fabricated. This device revealed high saturation current density of 350 mA/mm, transconductance of 95 mS/mm, and broad maximum transconductance region of 2.5 V with geometry of 2×100 μm2 at room temperature. In addition, very high breakdown voltage (26 V) and low leakage current (≤2 μA at −26 V) were achieved.

Secondary-ion mass spectrometry analysis in pseudomorphic GaAs/InGaAs/GaAs heterostructures utilizing a δ-doping superlattice

Significant improvement on two-dimensional electron gas (2DEG) concentration and mobility in a δ-doping superlattice GaAs/In0.25Ga0.75As/GaAs pseudomorphic heterostructures prepared by low-pressure metalorganic chemical vapor deposition have been demonstrated and discussed. The secondary-ion mass spectrometry profiles of these δ-doping superlattice structures were studied. The triple and double δ-doping superlattice heterostructures showed extremely high 2DEG concentrations of 8.8 (6.0) and 4.3 (2.5)×1012 cm−2 along with enhanced mobilities of 2710 (6500) and 3916 (18400) cm2/V s at 300 (77) K, respectively. The 2DEG concentrations, to our knowledge, are among the highest for previously reported pseudomorphic heterostructures with similar mobilities.

Transport properties of epitaxial lift-off films

Transport properties of epitaxially lifted-off (ELO) films were characterized using conductivity, Hall, and Shubnikov–de Haas measurements. A 10%–15% increase in the two-dimensional electron gas concentration was observed in these films as compared with adjacent conventional samples. We believe this result to be caused by a backgating effect produced by a charge buildup at the interface of the ELO film and the quartz substrate. This increase results in a significant decrease in the quantum lifetime in the ELO samples, by 17%–30%, but without a degradation in carrier mobility. Under persistent photoconductivity, only one subband was populated in the conventional structure, while in the ELO films the population of the second subband was clearly visible. However, the increase of the second subband concentration with increasing excitation is substantially smaller than anticipated due to screening of the backgating effect.

Room-temperature photoreflectance characterization of pseudomorphic GaAlAs/InGaAs/GaAs high electron mobility transistor structures including the two-dimensional electron gas density

Using contactless photoreflectance at 300 K the authors have characterized three pseudomorphic GaAlAs/InGaAs/GaAs high electron mobility transistor structures. The spectra from the InGaAs modulation-doped quantum well (MDQW) channel can be accounted for on the basis of a step-like two-dimensional density of states (screened exciton) and a Fermi level filling factor. A detailed lineshape fit makes it possible to evaluate the Fermi energy, and hence the two-dimensional electron gas concentration. Furthermore, other important parameters of the system such as built-in electric fields, In composition and well width of the InGaAs MDQW channel can be evaluated.

Very high two-dimensional electron gas concentrations with enhanced mobilities in selectively double-δ-doped GaAs/InGaAs pseudomorphic single quantum well heterostructures

In this letter a new type of GaAs/In0.25Ga0.75As/GaAs pseudomorphic single quantum well heterostructure by selectively double-δ-doping GaAs layers on both sides of the InGaAs channel grown by low-pressure metalorganic chemical vapor deposition is demonstrated. An extremely high two-dimensional electron gas concentration more than 1×1013 cm−2 with enhanced mobility of 2900 cm2/V s is achieved. This type of structure can easily break through the doping limitation in conventional heterostructures while keeping a high mobility.

Room-temperature determination of two-dimensional electron gas concentration and mobility in heterostructures

A technique for determination of room-temperature two-dimensional electron gas (2DEG) concentration and mobility in heterostructures is presented. Using simultaneous fits of the longitudinal and transverse voltages as a function of applied magnetic field, we were able to separate the parameters associated with the 2DEG from those of the parallel layer. Comparison with the Shubnikov–de Haas data derived from measurements at liquid helium temperatures proves that the analysis of the room-temperature data provides an excellent estimate of the 2DEG concentration. In addition we were able to obtain for the first time the room-temperature mobility of the 2DEG, an important parameter to device application. Both results are significantly different from those derived from conventional Hall analysis.

Deformation Potential in High Electron Mobility GaAs/GaAsAs Heterostructures

We have studied scattering mechanisms in high electron mobility GaAs/Ga0.7Al0.3 As heterostructures. We have performed measurements of two-dimensional electron gas concentration and mobility as functions of temperature and hydrostatic pressure. We focused our attention on two-dimensional scattering by acoustic phonons and we found that the magnitude of the deformation potential D for GaAs in the considered heterostructures is -12±1.0 eV. Contrary to the common assumptions about the pressure independence of D we have found that its absolute value decreases with applied hydrostatic pressure by about 10%/GPa.

Two-dimensional electron gas effects in the electromodulation spectra of a pseudomorphic Ga0.78Al0.22As/In0.21Ga0.79As/GaAs modulation-doped quantum well structure

We have studied the electroreflectance and photoreflectance spectra from a pseudomorphic Ga0.78Al0.22As/In0.21Ga0.79As/GaAs modulation-doped quantum well (MDQW) structure in the temperature range 79<T<304 K. The features from the InGaAs MDQW can be accounted for on the basis of a two-dimensional density of states and a Fermi level filling factor. A detailed line shape fit makes it possible to evaluate the Fermi energy, and hence the two-dimensional electron gas concentration (Ns), as well as other important parameters of the structure. Our value for Ns is in good agreement with a Hall measurement.

Characterization of Ultrahigh-Speed Pseudomorphic InGaAs/AlGaAs Inverted High Electron Mobility Transistors

A detailed characterization of ultrahigh-speed pseudomorphic InGaAs/AlGaAs inverted high electron mobility transistors (pseudomorphic I-HEMT's) is reported. Charge control analysis indicates that the pseudomorphic I-HEMT accomplishes improved transconductance (gm) and reduced short channel effects due to higher concentration and excellent confinement of two-dimensional electron gas (2DEG) in the InGaAs channel. The 0.2 µm gate pseudomorphic I-HEMT's reported here exhibit superior DC and RF performances, i.e., maximum transconductance (gmmax) of 565 mS/mm, cutoff frequency (fT) of 110 GHz and propagation delay times (τpd's) of 6.6 ps/gate at R.T. and 4.9 ps/gate at 120 K. Delay time analysis reveals the reduction of both the electron transit time (τtransit) and the channel charging time (τchannel) in the pseudomorphic I-HEMT, which are significant factors in the ultrahigh-speed performance. Higher electron saturation velocity (υs:2.1×107 cm/s) and improved gm are considered to contribute to the reduction in τtransit and τchannel.

Observation of metastable effects in pseudomorphic heterostructures InCaAs/InAlAs

Abstract The results of electrical transport measurements (Hall effect and resistivity) performed on In.65Ga.35As-In Al.48 As modulation doped heterostructure (with Si-doped InAlAs donor layer) and on In .52Al .48As thick layer are presented as a function of temperature (77–300K range) and hydrostatic pressure (up to 1300 MPa). At low temperature, the persistent photoconductivity (PPC) and metastable donor states occupation effects were observed. Due to the pressure induced decrease of the two-dimensional electron gas concentration the pronounced increase in the amplitude of observed effects was obtained. The above results are discussed in terms of DX-like centers and/or interface/surface states in the QW structure.

Variation of the mobility and the two-dimensional electron gas concentration with indium composition in δ-doped GaAs/InxGa1−xAs/GaAs pseudomorphic structures

The electron mobility and the two-dimensional electron gas (2DEG) concentration in different indium compositions (0.1<x<0.6) δ-doped GaAs/InxGa1−xAs/GaAs pseudomorphic structures grown by low-pressure metalorganic chemical vapor deposition are studied. The electron mobilities of a δ-doped GaAs layer are comparable to those of previous reports. Furthermore, the maximum mobility (5500 and 33 000 cm2/V s at 300 and 77 K, respectively) of the proposed pseudomorphic structure appears at x=0.37. Taking into account of strain and quantum effects, the variation trends of calculated 2DEG concentrations are in good agreement with the experimental results.

Extraction of FET parameters at low drain bias by taking into account the dependence of mobility on 2D electron gas concentration

An original method for the extraction of FET parameters at low drain bias is presented. It is based on a simple linear charge-control model and on a power-law dependence (with an exponent k > 0 for GaAs/AlGaAs MODFET's and k < 0 for Si-MOSFET's) of the low-field mobility μ on the two-dimensional electron gas (2DEG) concentration n s , valid in a certain range of gate voltages. Simple analytical expressions for the transfer characteristics I ds -V gs and g m -V gs at low drain bias are combined to extract reliable values of the threshold voltage V t , the power exponent k, the total parasitic series resistance (R s +R d ) and an important control parameter β'. We verified that the values of β' and k extracted on a test MODFET at 300 and 77 K agree very well with those deduced directly from Hall measurements on gated Hall-bridge structures. Our analysis is also applied to extract MOSFET parameters at room temperature.

The δ-Doped In0.25Ga0.75As/GaAs Pseudomorphic High Electron Mobility Transistor Structures Prepared by Low-Pressure Metal Organic Chemical Vapor Deposition*

The (800 Å)GaAs/In0.25Ga0.75As/(0.5 µm) GaAs strain layers with δ-doping in the GaAs cap layer were fabricated without using AlGaAs layers. Triethylgallium (TEG), trimethylindium (TMI), arsine (AsH3) and silane (SiH4) were used as the sources and dopants in these structures grown by the low-pressure metalorganic chemical vapor deposition (LP-MOCVD) technique for the first time to study these structures. The δ-doping was obtained by a stop-growth process by which a very thin and heavily doped layer (1.86×1013 cm-2) can be realized. For the (δ-doped) GaAs/In0.25Ga0.75As/GaAs high electron mobility transistor (HEMT) structures, the concentration of two-dimensional electron gas (2-DEG) and the Hall mobilities were investigated. Experimental results show that a structure with (80 Å) In0.25Ga0.75As as the active channel and with a 70 Å spacer layer revealed the highest 2-DEG concentration and mobility product of 5.5times1016 V-1·s-1 at a temperature of 77 K. These structures are easy to achieve by the LP-MOCVD mechod and are promising for high-performance field effect transistors (FETs).