Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With particular semiconductor material
Reexamination Certificate
2000-11-28
2003-02-04
Cao, Phat X. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With particular semiconductor material
C257S094000, C257S101000, C257S102000
Reexamination Certificate
active
06515313
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to light emitting compound semiconductor crystals grown on a polar surface and, more particularly, to reduction or cancellation of their naturally occurring polarization-induced charges to improve emission efficiency.
2. Description of the Related Art
Most semiconductor light emitters have a double heterostructure structure that includes an active or light-generating layer grown between two cladding layers. The various layers of the double heterostructure are fabricated from more than one material. One cladding layer is n-type, which means it contains excess free electrons, and one cladding layer is p-type, which means it contains excess holes. In general, the cladding layers have larger bandgaps than the active layer. This causes injected electrons and holes to be confined within the active layer, encouraging efficient recombination of free carriers through spatial localization within the active layer to produce light. In addition, laser diode (LD) emitters also have separate light confining layers, typically comprised of a material with an even wider bandgap, surrounding a double heterostructure. Double heterostructure semiconductor devices are described in numerous publications, including O'Shea et al,
Introduction to Lasers and Their Applications
, Addison-Wesley Publishing Company, December 1978, pages 166-167.
In such structures, polarization-induced charges occur when the material composition varies in a polar direction of its basic crystal structure. A polar direction is defined as any crystal direction not orthogonal to the polarization vector, {overscore (P)}, of the crystal. This is especially true for materials whose crystal bonds are naturally directional and even slightly ionic, such as in III-V or II-VI semiconductors. These charges can be strain-related (piezoelectric) in the case of lattice mismatched materials, composition related (spontaneous) due to differences in the ionic strengths of bonds in different materials, or a combination of the two. The induced charges cause electric fields or potential gradients that have the same effect on free carriers as external fields. The phenomenon is discussed in a number of publications, including Bernardini et al, “Spontaneous polarization and piezoelectric constants of III-V nitrides,”
American Physical Society Journal
, Physics Review B, Vol. 56, No. 16, 1997, pages R10 024 to R10 027, and Takeuchi et al, “Quantum-Confined Stark Effect due to Piezoelectric Fields in GaInN Strained Quantum Wells,”
Japanese Journal of Applied Physics
, Vol. 36, Part 2, No. 4, 1997, pages L382-L385. The magnitudes of such fields have been estimated to be as high as 2.5×10
6
V/cm for nitride heterostructures grown on a polar surface of a crystal, Bykhovski et al., “Elastic strain relaxation and piezo-effect in GaN—AlN, Gan—AlGaN and GaN—InGaN superlattices”,
Journal of Applied Physics
, Vol. 81, No. 9, 1997, pages 6332-6338.
Polarization-induced charges should be taken into account when considering the electrical characteristics of heterostructures grown on crystal polar surfaces. Crystal layers grown along the 0001 orientation in the case of wurtzite GaN crystal, or along the 111 orientation in the case of zincblende GaAs crystals, are two examples of crystal polar surfaces. The Bravais lattice of the wurtzite structure is hexagonal, with the axis perpendicular to the hexagons usually labeled as the c-axis or the 0001 orientation. Along this axis the structure can be thought of as a sequence of atomic layers of the same element (e.g. all Gallium or all Nitrogen) built up from regular hexagons. Due to this uniformity, each layer (or surface) is polarized and possesses either a positive or a negative charge, generating a dipole across the atomic layers. The charge state of each layer depends upon its constituent atoms. Other examples of crystal planes with various growth directions may be found in Streetman,
Solid State Electronic Devices
, 2nd ed., Prentice-Hall, Inc., 1980, pages 1-24, and Shuji Nakamura et al, “The Blue Laser Diode, GaN Based Light Emitters and Lasers,” Springer, 1997, pages 21-24.
Until recently, internal polarization fields associated with the active and cladding regions of a light emitting heterostructure have not posed significant problems. This was because light emitting diodes (LEDs) based on the more established Al—Ga—In—As—P material system have typically been grown on a non-polar crystal surface (in particular the 001 zincblende surface). Recently, however, there has been considerable work in light emitters based on the Al—Ga—In—N (“nitride”) materials system, mostly grown along the 0001 orientation of wurtzite crystal, which is a highly polar surface. Nevertheless, nitride double heterostructures have followed conventional non-polar designs.
FIG. 1A
is a sectional view schematically illustrating a typical conventional nitride double heterostructure semiconductor grown in a polar direction. The illustrated substrate layer
1
may be any material suitable for growing nitride semiconductors, including spinel (MgAl
2
O
4
), sapphire (Al
2
O
3
), SiC (including 6H, 4H, and 3C), ZnS, ZnO, GaAs, AlN and GaN. The substrate thickness typically ranges from 100 &mgr;m to 1 mm. A buffer layer
2
on the substrate
1
can be formed of AlN, GaN, AlGaN, InGaN or the like. The buffer layer facilitates possible lattice mismatches between the substrate
1
and an overlying conductive contact layer
3
. However, the buffer layer
2
may be omitted if the substrate has a lattice constant approximately equal to that of the nitride semiconductor. The buffer layer
2
may also be omitted with some nitride growth techniques. Depending upon the material composition, the buffer layer energy bandgap may range from 2.1 eV to 6.2 eV, with a thickness of about 0.5 &mgr;m to 10 &mgr;m.
The n-type contact layer
3
is also typically formed from a nitride semiconductor, preferably GaN or InGaN with a thickness ranging from 0.5 &mgr;m to 5.0 &mgr;m, and a bandgap of approximately 3.4 eV for GaN and less for InGaN (depending upon the Indium concentration). A lower n-type or undoped cladding layer
4
on the conductive layer
3
conventionally comprises GaN or AlGaN, with a bandgap of 3.4 eV for GaN and greater for AlGaN (depending upon the Al concentration). Its thickness can range from 1 nm to 100 nm.
Nitride double heterostructures typically employ InGaN as an active region
5
over the lower cladding layer, with a thickness of 1 nm to 100 nm. The bandgap of this layer is typically 2.0 eV, but may vary depending upon the Indium concentration. A top p-type or undoped cladding layer
6
over the active region is generally comprised of AlGaN or GaN, with a thickness and bandgap energy similar to that of the lower n-type cladding layer
4
. A p-type GaN conductive contact layer
7
on the cladding layer
6
has an energy bandgap of about 3.4 eV and a thickness of about 10 nm to 500 nm. In general, provided the structure is grown on a polar direction such as the 0001, a polarization-induced sheet charge occurs at the interface between layers due to different constituent materials. Of particular concern for the operation of a light emitter are the polarization-induced charge sheets adjacent to the active region
5
.
With the compound semiconductor illustrated in
FIG. 1A
, a negative polarization-induced charge sheet density &sgr;
1
, with a magnitude such as 10
13
electrons/cm
2
, is typically formed at the interface between the active region
5
and the lower cladding layer
4
. A positive charge sheet density &sgr;
2
of similar magnitude is formed at the interface between the active region
5
and the upper cladding layer
6
. The polarities of these charges depend upon the bonds of the crystal layers, which as mentioned above are directional and slightly ionic. In general, the density of a charge sheet will depend upon both a spontaneous factor arising from compositional differences between the two layers, and a piezoelectric strain arising from
Ibbetson James
Thibeault Brian
Cao Phat X.
Cree Lighting Company
Koppel, Jacobs Patrick & Heybl
Wille Douglas A.
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