Polarization field enhanced tunnel structures

A novel tunnel structure is described that enables tunnel diode behavior to be exhibited even in material systems in which extremely heavy doping is impossible and only moderate or light doping levels may be achieved. In one aspect, the tunnel heterostructure includes a first semiconductor layer, a second semiconductor layer, and an intermediate semiconductor layer that is sandwiched between the first and second semiconductor layers and forms first and second heterointerfaces respectively therewith. The first and second heterointerfaces are characterized by respective polarization charge regions that produce a polarization field across the intermediate semiconductor layer that promotes charge carrier tunneling through the intermediate semiconductor layer. In another aspect, the invention features a semiconductor structure having a p-type region, and the above-described heterostructure disposed as a tunnel contact between the p-type region of the semiconductor structure and an adjacent n-type region.

TECHNICAL FIELD

This invention relates to polarization field enhanced tunnel structures.

BACKGROUND

Many different tunnel structures have been developed, including tunnel diode structures. A typical tunnel diode is formed from a p-n junction in which both n and p sides are degenerate (i.e., very heavily doped with impurities). The high doping levels bend the conduction and valence bands over a short distance (e.g., on the order of 10 nm, or less) such that conduction band electron states at the Fermi level on the n side are aligned with valence band states at the Fermi level on the p side. This phenomenon allows charge carriers to tunnel across the depletion layer upon application of an arbitrarily small forward or reverse bias across the p-n junction. Beyond a particular forward bias level, the available charge carrier tunnel states do not overlap, resulting in a region of negative differential resistance (i.e., where the current exhibits a decrease in value when the value of the voltage is increased).

In general, several conditions must be satisfied to achieve such a tunnel diode structure. For example, the n and p sides of the tunnel structure must be degenerately doped. This condition requires a combination of heavy doping levels and low dopant ionization energies. In addition, the tunnel distance must be sufficiently short to enable charge carriers to tunnel across the depletion region with a relatively high probability. This condition requires very heavy doping levels. One or more of these tunneling conditions cannot be satisfied in certain semiconductor material systems. For example, in many semiconductors (especially wide bandgap semiconductors, such as III-V nitride and II-VI semiconductors) heavy doping of the n side or the p side, or both, is difficult or impossible to achieve. In addition, in these semiconductor material systems, dopant ionization energies often are relatively high, especially for p-type dopants. The combined effect of these limitations makes it difficult or impossible to fabricate tunnel diode structures that operate at moderate reverse voltages, much less operate at nearly zero voltage.

SUMMARY

The invention provides a novel tunnel structure that enables tunnel diode behavior to be exhibited even in material systems in which extremely heavy doping is impossible and only moderate or light doping levels may be achieved.

In one aspect, the invention features a heterostructure that includes a first semiconductor layer, a second semiconductor layer, and an intermediate semiconductor layer that is sandwiched between the first and second semiconductor layers and forms first and second heterointerfaces respectively therewith. The first and second heterointerfaces are characterized by respective polarization charge regions that produce a polarization field across the intermediate semiconductor layer that promotes charge carrier tunneling through the intermediate semiconductor layer.

Embodiments of the invention may include one or more of the following features.

The intermediate semiconductor layer preferably has a thickness (D) that enables charge carriers to tunnel through the intermediate semiconductor layer with a current density greater than 1 A/cm2. In some embodiments, the polarization field may have a magnitude (ξp) with a value that is on the order of (Ec,1−Ev,2)/(q·D), wherein Ec,1is a relative conduction band energy at the first heterointerface, Ev,2is a relative valence band energy at the second heterointerface, q is a unit carrier charge, and D is the thickness of the intermediate semiconductor layer. The polarization field may have a magnitude (ξp) that is sufficient to align an occupied conduction band state at the first heterointerface with an unoccupied valence band state at the second heterointerface.

The first semiconductor layer preferably is doped n-type and the second semiconductor layer preferably is doped p-type. The polarization field preferably enhances a dopant-induced drift field that is produced between the first and second semiconductor layers.

The first and second semiconductor layers may be formed from the same semiconductor material. For example, in one embodiment, the first and second semiconductor layers may be formed from GaN and the intermediate semiconductor layer may be formed from AlGaN. In another embodiment, the first and second semiconductor layers may be formed from GaN and the intermediate semiconductor layer may be formed from InGaN.

The polarization charge regions may form as a result of spontaneous polarization differences between the two materials, or piezoelectric polarization, or both.

In those embodiments exhibiting spontaneous polarization, the first, second and intermediate semiconductor layers are characterized by crystallographic structures allowing spontaneous polarization charge formation at the first and second heterointerfaces. In some of these embodiments, each of the first, second and intermediate semiconductor layers may have a hexagonal crystallographic structure. The first, second and intermediate semiconductor layers may be formed from a III-V nitride semiconductor material, such as GaN, AlGaN, InGaN, AlN, InN, InAlN. In other embodiments exhibiting spontaneous polarization, each of the first, second and intermediate semiconductor layers may be formed from a II-VI semiconductor material.

In another aspect, the invention features a semiconductor structure having a p-type region, and the above-described heterostructure disposed as a tunnel contact junction between the p-type region of the semiconductor structure and an adjacent n type region.

In some embodiments in accordance with this aspect of the invention, the semiconductor structure includes a light emitting region.

The inventive tunnel heterostructures described herein may be used advantageously in conventional tunnel diode applications (such as microwave oscillators) that depend on the negative differential resistance under forward bias. They can also be used under reverse bias as “leaky” diodes to provide low-voltage drop contacts in material systems in which such contacts are difficult or impossible to achieve. Such low-voltage drop contacts may be used to reduce forward voltages, reduce self-heating and increase power efficiency of semiconductor devices in general, and may be used to reduce the lasing thresholds of semiconductor light-emitting diodes and semiconductor lasers in particular. In addition, the inventive tunnel heterostructure may be used in semiconductor devices (e.g., light emitting semiconductor devices) as a hole source. In this way, the invention allows low mobility and high resistance p-type conductivity channels to be replaced by high mobility and low resistance n-type conductivity channels to improve device performance.

DETAILED DESCRIPTION

Referring toFIG. 1, in one embodiment, a polarization field enhanced tunnel heterostructure10includes a first semiconductor layer12, a second semiconductor layer14, and an intermediate semiconductor layer16. Intermediate semiconductor layer16is sandwiched between the first and second semiconductor layers12,14, and forms first and second heterointerfaces18,20respectively therewith. First semiconductor layer12may be doped n-type, and second semiconductor layer14may be doped p-type. Intermediate semiconductor layer16has a thickness (D) that is sufficiently thin to enable charge carriers to tunnel through intermediate semiconductor layer16with a substantial probability. For example, in some embodiments, intermediate semiconductor layer16may have a thickness that is sufficiently thin to enable charge carriers to tunnel through intermediate semiconductor layer16with a current density greater than 1 A/cm2. In some embodiments, intermediate semiconductor layer16has a thickness that is on the order of about 10 nm, or less. Heterostructure10may be formed on a semiconductor wafer (e.g., a silicon wafer or a gallium arsenide wafer), a magnesium oxide substrate, a sapphire substrate, the top surface of a single semiconductor layer, or the top surface of a multilayer structure that includes, for example, a semiconductor device (e.g., a light-emitting diode or a semiconductor laser) that is formed on a semiconductor wafer.

As shown inFIG. 2, at zero bias heterostructure10includes a depletion region with a positive space charge region22and a negative space charge region24that form as a result of the n and p doping of first and second semiconductor layers12,14, respectively. In addition, the first and second heterointerfaces18,20include positive and negative polarization charge regions26,28, respectively. Polarization charge regions26,28may form from differences in spontaneous polarization between adjacent layers, or from piezoelectric polarization due to strain in one or more of the layers, or from a combination of both spontaneous polarization and piezoelectric polarization. The more abrupt the heterointerface, the narrower the polarization charge region will be. The invention includes both abrupt and graded heterointerfaces. Piezoelectric polarization results from strain due to differences in lattice constant among layers12,14,16. For example, the strain that is produced in the InGaN layer of a GaN/InGaN/GaN heterostructure produces a substantial piezoelectric field across the InGaN intermediate layer.

In general, intermediate semiconductor layer16is formed from a material that is different from the constituent materials of the first and second semiconductor layers12,14. First and second semiconductor layers12,14may be formed from the same or different materials. First, second and intermediate semiconductor layers12,14,16may be formed from any semiconductor material system that exhibits internal polarization fields, including semiconductor materials having a hexagonal crystallographic structure, III-V nitride semiconductors (e.g., GaN, AlGaN, InGaN, AlN, InN, InAlN), and II-VI semiconductors (e.g., ZnSe, ZnSSe, ZnCdSe, ZnHgSe, HgSe, ZnTeSe). For example, in one embodiment, heterostructure10may be formed from GaN/InGaN/GaN.

As explained in detail below, polarization charge regions26,28produce a polarization field (ξp) across intermediate semiconductor layer16that promotes charge carrier tunneling through intermediate semiconductor layer16.

Referring toFIGS. 3A,3B,3C and4, in one embodiment, the polarization field (ξp) has a magnitude that is sufficient to enable tunneling at infinitesimal applied bias. In particular, the polarization field magnitude (together with the dopant-induced drift field, ξd) is sufficient to align conduction band states near the Fermi level (EF) at the first heterointerface18with valence band states the Fermi level (EF) at the second heterointerface20. At zero bias (FIG.3A), the number of electrons that tunnel from the conduction band of layer12to the valence band of layer14is as many as the number of electrons that tunnel in the opposite direction. Therefore, at zero bias, the tunneling current is zero (see point A in FIG.4). When a relatively small forward bias (Vf) is applied (FIG.3B), however, the quasi Fermi levels are split by an amount equal to qVf. There are now a large number of empty states in layer14available for electrons to tunnel from the conduction band in the n-doped first semiconductor layer12. As a result, tunneling current may flow (see point B in FIG.4). When the forward bias is increased further, the available charge carrier tunneling states do not overlap and no states are available for direct tunneling (FIG.3C). Under such conditions, the tunneling current drops (see point C in FIG.4). The subsequent rise in the diode current is related to normal diode conduction mechanisms. Under reverse bias conditions, electrons tunnel from occupied states in the valence band of the p-type second semiconductor layer14into empty states of the conduction band of the n-type first semiconductor layer12. The result is a “leaky” (or backward) diode with a relatively small voltage drop. As shown inFIG. 4, the resulting current-voltage characteristic is similar to that of a conventional tunnel diode. This polarization field enhanced tunnel heterostructure may be used for low-power microwave applications, such as local oscillation and frequency-locking circuits, as well as rectification of small signals, microwave detection, and mixing.

As mentioned above, in the embodiment ofFIGS. 3A-4, the polarization field has a magnitude (ξp) that is sufficient to align conduction band states near EFat first heterointerface18with valence band states near EFat second heterointerface20. That is, ξphas a value that is on the order of (Ec,1−Ev,2)/(q·D), where Ec,1is a relative conduction band energy at first heterointerface18, Ev,2is a relative valence band energy at second heterointerface20, q is a unit carrier charge, and D is the thickness of the intermediate semiconductor layer16. In particular, in this embodiment, ξphas a value that is greater than (Ec,1−Ev,2)/(q·D)−ξd, where ξdis the value of the dopant-induced drift field through the intermediate semiconductor layer as shown in FIG.3A. Consequently, although the intermediate semiconductor layer thickness (D) should be thin enough to enable charge carriers to tunnel therethrough, it should not be thinner than (Ec,1−Ev,2)/(q·(ξp+ξd)).

Referring toFIGS. 5A,5B and6, in some embodiments, the polarization field magnitude (together with the dopant-induced drift field) may be insufficient to align an occupied conduction band state at the first heterointerface18with an unoccupied valence band state at the second heterointerface20at zero applied bias (FIG.5A). Consequently, such a heterostructure does not exhibit the negative differential resistance that is characteristic of the heterostructure discussed above in connection withFIGS. 3A-4. However, such a heterostructure still may be operated under reverse bias conditions to provide a “leaky” (or backward) diode with a relatively small voltage drop. Thus, as shown inFIG. 5B, only a relatively small reverse bias (Vr) is needed to initiate electron tunneling from occupied states in the valence band of the p-type second semiconductor layer14into empty states of the conduction band of the n-type first semiconductor layer12. As shown inFIG. 6, under small bias conditions, the current in the reverse direction is larger than the current in the forward direction. This polarization field enhanced tunnel heterostructure may be used for rectification of small signals, microwave detection, and mixing.

Although the above discussion focuses on electron tunneling current, holes also may tunnel through intermediate semiconductor layer16. In most cases, however, holes are heavier than electrons and, as a result, holes have a much lower tunneling probability than electrons. In addition, in the above discussion, the bandgaps of the first, second and intermediate semiconductor layers12,14,16are shown as having the same magnitude for illustrative purposes. In actual embodiments, however, the bandgap of intermediate semiconductor layer16would be different from the bandgaps of first and second semiconductor layers12,14. In many applications, it will be advantageous for layer16to have a smaller bandgap in order to increase the tunneling current. The bandgaps of first and second semiconductor layers12,14on the other hand, may be the same or different, depending upon their respective material compositions.

The polarization field enhanced tunnel heterostructures ofFIGS. 3A-4and5A-6may be used to provide low-voltage drop contacts between adjacent n-type and p-type conducting layers in device structures in which it is difficult to achieve low-resistance contacts to p-type conducting layers. For example, in many semiconductors, the maximum achievable p-type conductivity is significantly worse than the n-type conductivity. The relatively poor p-type conductivity limits device performance by increasing the access resistance to the active regions of the device. In such situations, significant performance improvements may be realized by incorporating a leaky p-n junction diode into the structure to replace the p-type conductivity with an n-type conducting layer. By incorporating one of the above-described polarization enhanced tunnel heterostructures into such a material system, a low-voltage drop p contact junction may be created even when extremely heavy doping is impossible and only moderate or light doping levels may be achieved. For example, polarization field enhanced tunnel heterostructure10may be implemented as a leaky diode to provide a low-voltage drop p contact junction in many different devices, including light emitting diodes and semiconductor lasers (e.g., an edge-emitting laser or a vertical cavity surface emitting laser).

As shown inFIG. 7, in one embodiment, polarization field enhanced tunnel heterostructure10may operate as a tunnel contact junction in a semiconductor laser52. Laser52may be formed from InGaN and AlGaN alloys. In this embodiment, a GaN buffer layer54may be formed on a sapphire substrate56. An n-type GaN contact layer58may be formed between the GaN buffer layer54and polarization field enhanced tunnel heterostructure10. Polarization field enhanced tunnel heterostructure10includes an n-type GaN bottom layer60, an InGaN intermediate layer62, and a p-type GaN top layer64. A p-type AlGaN cladding layer66and a p-type GaN waveguide layer68may be formed over the p-type GaN top layer64. An InGaN multi-quantum well layer70may be disposed between the p-type GaN waveguide layer68and an n-type GaN waveguide layer72and an overlying n-type AlGaN cladding layer76. An n-type GaN contact layer77is disposed over the AlGaN cladding layer76. A silicon dioxide insulating layer78is deposited over the laser structure. A via80is formed through silicon dioxide layer78to expose a portion of n-type GaN contact layer77. Finally, a metal electrode82is deposited over n-type GaN contact layer58and a metal electrode84is deposited over n-type GaN contact layer77. In this embodiment, tunnel diode heterostructure10provides a low-voltage contact junction between the p side of semiconductor laser52and an n-type conductivity channel provided by contact layer58. In this way, tunnel diode heterostructure10avoids the need for a p-type current-carrying layer, which often is characterized by a low carrier density, a low mobility, and a high resistance for lateral conduction. In addition, tunnel diode heterostructure10allows design of a laser structure that avoids the high contact resistance between p-type layers and metal contacts.

In other embodiments, polarization field enhanced tunnel heterostructure10may operate as a tunnel contact junction in one or more other regions of a light emitting diode or a semiconductor laser device. For example, tunnel heterostructure10may be used as a hole source to avoid p-type layers, such as p-type AlGaN layers, which are poor electrical conductors. Tunnel heterostructure10also may be used as a tunnel contact between a p-type layer and a metal contact.

In other embodiments, the constituent layers of laser42may be formed from different III-V nitride semiconductor materials or from II-VI semiconductor materials. In addition, polarization field enhanced tunnel heterostructure10may be implemented as a tunnel contact to provide a low-voltage drop p-type contact in other semiconductor devices, including light-emitting diodes and vertical cavity surface emitting lasers (VCSELs).

Other embodiments are within the scope of the claims.