VERTICAL CAVITY SURFACE-EMITTING LASER EPITAXIAL STRUCTURE HAVING A CURRENT SPREADING LAYER

A vertical cavity surface-emitting laser epitaxial structure having a current spreading layer is disclosed. The vertical cavity surface-emitting laser epitaxial structure includes a substrate, a first epitaxial region on the substrate, an active region on the first epitaxial region, and a current spreading layer disposed in the first epitaxial region. The current spreading layer includes an N-type dopant, and the N-type dopant is selected from a group consisting of Si, Se, and the combination thereof. The current spreading layer does not directly contact the active region.

RELATED APPLICATIONS

This application claims priority to Taiwanese Application Serial Number 111110938, filed Mar. 23, 2022, and Taiwanese Application Serial Number 111122449, filed Jun. 16, 2022, all of which are herein incorporated by reference in their entireties.

BACKGROUND

Field of Invention

The present disclosure relates to a vertical cavity surface-emitting laser (VCSEL) epitaxial structure. More particularly, the present disclosure relates to a VCSEL epitaxial structure with a current spreading layer, that is suitable for fabrication high-density VCSEL array.

Description of Related Art

A vertical cavity surface-emitting laser (VCSEL) epitaxial structure is used to manufacture VCSEL device, including VCSEL diode or a VCSEL array. The area of a bottom epitaxial region of the VCSEL array is multiple times to thousands of times larger than the area of the VCSEL die. If the current distribution area in the bottom epitaxial region of the VCSEL array is not enlarged accordingly, the resistance of the bottom epitaxial region would be huge, and the power conversion efficiency of the VCSEL array is poor.

US Patent publication No. 2018/0175587A1 discloses a VCSEL array. However, the current spreading layer210is disposed over the multiple quantum well layer216, rather than below the multiple quantum well layer216.

U.S. Pat. No. 6,549,556B1 discloses a current-spreading layer116below the active region115. However, U.S. Pat. No. 6,549,556B1 is not related to a VCSEL array, the area of the bottom epitaxial region thereof is not too large for the current to spread evenly. U.S. Pat. No. 6,549,556B1 also fails to disclose that the current-spreading layer116is disposed in a bottom cavity. It must be noted, the bottom cavity of U.S. Pat. No. 6,549,556B1 is made of dielectric material, and no current would flow into the bottom cavity of U.S. Pat. No. 6,549,556B1.

SUMMARY

According to some embodiments of the disclosure, a vertical cavity surface-emitting laser epitaxial structure having a current spreading layer is disclosed. The vertical cavity surface-emitting laser epitaxial structure includes a substrate, a first epitaxial region on the substrate, an active region on the first epitaxial region, and a current spreading layer disposed in the first epitaxial region. The current spreading layer includes an N-type dopant, and the N-type dopant is selected from a group consisting of Si, Se, and the combination thereof. The current spreading layer does not directly contact the active region.

According to some embodiments of the disclosure, a vertical cavity surface-emitting laser epitaxial structure having a current spreading layer is disclosed. The vertical cavity surface-emitting laser epitaxial structure includes a substrate, a first epitaxial region on the substrate, and a current spreading layer. The first epitaxial region includes a bottom distributed Bragg reflector (DBR) layer. The current spreading layer is disposed between the bottom DBR layer and the substrate. The current spreading layer includes an N-type dopant, and the N-type dopant is selected from a group consisting of Si, Se, and the combination thereof.

Compared to the VCSEL array without the current spreading layer disposed in the bottom epitaxial region, the current distribution area herein is larger. Therefore, the resistance of the bottom epitaxial region such as resistance of the bottom DBR layer can be reduced, and the power conversion efficiency of the VCSEL (array) is enhanced. Additionally, as the current spreading layer is spaced from the active region by a suitable distance, so the problem of the current spreading layer is absorbing the light emitted from the active region can be prevented, thereby keeping the light-emitting efficiency of the VCSEL array as bigger as VCSEL die.

The situation of the current spreading layer being spaced from the active region by a suitable distance includes disposing at least one semiconductor layer such as a spacer layer, a tunnel junction layer or other suitable semiconductor layer between the active region and the current spreading layer.

DESCRIPTION OF THE EMBODIMENTS

The drawings illustrate embodiments of the disclosure and, together with the description, serve to explain the principles of the present disclosure. For better understanding the concept of the present disclosure, only parts of the structure of the laser diode is illustrated, and the laser diode is not limited to be consisted of the mentioned elements. The thickness ratios of the layers in the drawing are for example, the thicknesses of the layers can be adjusted according to real requirements.

It will be understood that when an element is referred to as being “on” another element, it can be directly on the other element or intervening elements may be present therebetween. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Reference will now be made in detail to the present embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.

Various embodiments are provided in the present disclosure. The exemplary terms “in some embodiments” means that the mentioned particular features, structures, or limitations can be included in at least one of the embodiments of the present disclosure. The exemplary terms “in some embodiments” is not necessary be the same embodiment.

Additionally, the term “layer” can be a single layer or a multi-layer. A part of an epitaxial layer can be one layer or adjacent layers of the epitaxial layer.

The VCSEL (array) discussed in the present disclosure is a vertical resonant cavity surface-emitting laser (array), which can be a top-emitting type or bottom-emitting type vertical resonant cavity surface-emitting laser (array).

Generally, the laser diode can optionally include a buffer layer based on the real requirement. In some embodiments, the buffer layer can be made of the same material as that of the substrate. Disposing a buffer layer or not is not the point of the technical feature or the advantage described in the following embodiments. Although the exemplary embodiments described in the following description are related to the laser diode having the buffer layer, the concept of the following embodiments can be also applied to the laser diode without the buffer layer.

Reference is made toFIG.1a, which is a schematic view of a VCSEL epitaxial structure according to some embodiments of the disclosure. The VCSEL epitaxial structure100includes a substrate10, an ohmic contact layer11, a first epitaxial region E1, an active region A, a second epitaxial region E2, a current spreading layer18, and a top ohmic contact layer19. The first epitaxial region E1includes a buffer layer12, a bottom DBR layer14, a spacer layer16and the current spreading layer18. The active region A includes one or more active layers, an active layer may include a quantum well layer or a multiple quantum well layer. The top ohmic contact layer19is disposed above the second epitaxial region E2. The epitaxial structure or the epitaxial layers herein can be epitaxially grown on the substrate by MOCVD.

In the embodiments shown inFIG.1aorFIG.1b, the current spreading layer18doped with an N-type dopant is inserted in the bottom DBR layer14. The bottom DBR layer14includes alternating pairs of a low refraction layer and a high refraction layer. As shown inFIG.1a, the current spreading layer18is disposed in the bottom DBR layer14and is located near the active region A. The current spreading layer18can laterally spread current to the layers below the bottom DBR layer14, thereby reducing the resistance of the bottom DBR layer14. The lateral direction of the disclosure is parallel (or substantially parallel) to the epi plane direction.

As shown inFIG.1a, the ohmic contact layer11is disposed below the substrate10. Namely, the substrate10is disposed between the ohmic contact layer11and the buffer layer12. Alternatively, as shown in the VCSEL epitaxial structure101ofFIG.1b, the ohmic contact layer11is disposed between the buffer layer12and the bottom DBR layer14. According to requirements, the ohmic contact layer11can be inserted in the buffer layer12or the bottom DBR layer14(not shown).

Each of the epitaxial layers in the first epitaxial region E1is an N-type material layer. Each of the epitaxial layers in the second epitaxial region E2is a P-type material layer. Alternatively, a tunnel junction layer can be disposed in the second epitaxial region E2such that the second epitaxial region E2includes a P-type epitaxial layer and N-type epitaxial layer.

As shown in the VCSEL epitaxial structure102ofFIG.2, the bottom DBR layer14includes an N-type portion141, a tunnel junction layer143, a P-type portion145, and the current spreading layer18, wherein the current spreading layer18is inserted in the N-type portion141, and the tunnel junction layer143is between the N-type portion141and the P-type portion145. The N-type portion141includes a plurality of N-type alternating pairs, the P-type portion145includes a plurality of P-type alternating pairs, and the tunnel junction layer143is disposed between the N-type portion141and the P-type portion145. Because the P-type alternating pair has a lower interface resistance, the resistance of the bottom DBR layer14can be further reduced.

As shown in the VCSEL epitaxial structure103ofFIG.3, the tunnel junction layer143is disposed between the spacer layer16and the bottom DBR layer14. In the VCSEL epitaxial structure103ofFIG.3, each epitaxial layer between the tunnel junction layer143and the substrate10is N-type material, and the spacer layer16between the tunnel junction layer143and the active region A is P-type material. The current spreading layer18is inserted in the bottom DBR layer14and is located near the active region A, wherein the direction of the current is from the active region A towards the substrate10, but not limited thereto. Therefore, the current can laterally spread in most of the layers in the bottom DBR layer14, and the current distribution area range of the bottom DBR layer14is larger.

In another aspect, it is not easy to laterally spread the current in the substrate10when the current passes the substrate10. By directly or indirectly disposed the current spreading layer18on the substrate10, the current distribution area range in the substrate1014is larger. Details thereof are described in the following embodiments. It is noted that the embodiments described below and above can be modified and/or matched.

As shown in the VCSEL epitaxial structure104ofFIG.4, as the substrate is a conductive substrate10a, the ohmic contact layer11can be disposed below the conductive substrate10aor disposed over the conductive substrate10a(not shown). In this embodiment, the current spreading layer18is directly disposed on the conductive substrate10a. A doping concentration of a portion of the current spreading layer18is greater than a doping concentration of the conductive substrate10a.

As shown in the VCSEL epitaxial structure105ofFIG.5, the substrate is a semi-insulating substrate10b, and the ohmic contact layer11can be disposed over the semi-insulating substrate10b. Preferably, the current spreading layer18is disposed in the ohmic contact layer11and is located near the semi-insulating substrate10b. As shown inFIG.5, the current spreading layer18is disposed between the buffer layer12and the ohmic contact layer11. In other word, the current spreading layer18is a lower portion of the ohmic contact layer11, and an upper portion of the ohmic contact layer11is doped with Si, Se, or other suitable element.

In some other embodiments, the current spreading layer18is the upper portion of the ohmic contact layer11, and the lower portion of the ohmic contact layer11is doped with Si, Se, or other suitable element. In yet some other embodiments, the current spreading layer18is a middle portion of the ohmic contact layer11, and the upper and/or lower portions of the ohmic contact layer11is/are doped with Si, Se, or other suitable element.

As shown in the VCSEL epitaxial structure106ofFIG.6, two current spreading layers18and181are disposed in the first epitaxial layer E1. The current spreading layer18is inserted in the bottom DBR layer14, and the current spreading layer181is disposed between the substrate10and the bottom DBR layer14. In addition, a top DBR layer14ais disposed in the second epitaxial region E2.

In any one of aforementioned embodiments discussed inFIG.1atoFIG.6, preferably, the material of the current spreading layer is GaAs, GaAsSb, InGaAs, InGaAsSb, InGaP, AlGaAs, GaAsP, AlGaInP, or combinations thereof, and the current spreading layer is doped with Se, Si, or other suitable element. In some embodiments, the substrate is made of GaAs, and the N-type current spreading layer includes a material selected from a group consisting of GaAs, GaAsP, InGaP, InGaPN, InGaPSb, InGaPBi, InGaAsP, InAlGaP, InAlGaPN, InAlGaPBi, InAlGaPSb, AlGaAs, AlGaAsP, and AlGaAsSb, in which an Al percentage of the AlGaAs, AlGaAsP, or AlGaAsSb is less than or equal to 30%. If the Al percentage of the N-type current spreading layer is smaller than 30%, the carrier barrier would be lowered, thereby increasing the resistance. In some embodiments, the substrate is made of InP, and the N-type current spreading layer includes a material selected from a group consisting of InGaAs, InGaAsSb, GaAsSb, InP, InGaAsP, InAlAs, InAlGaAs, InAlAsSb, InAlGaAsSb, and AlAsSb.

As a result, the resistance of the VCSEL array having the current spreading layer is less than the conventional VCSEL array. Especially when the density of the VCSEL array is higher, the lateral current spreading effect provided by the current spreading layer is still considerable. Therefore, the power conversion efficiency of the high density VCSEL array is improved, and the optical output power or characteristics of the VCSEL array would not be mitigated.

By appropriately selecting the materials for the current spreading layer and the bottom DBR layers, the conduction band discontinuity (ΔEc) between them can be reduced, resulting in a small carrier barrier and the low resistance wherein the bottom DBR layer is made of GaAs and/or AlGaAs Furthermore, comparing to the P-type current spreading layer, the N-type current spreading layer has lower light-absorbing ability, thus the influence of the optical output power of the VCSEL array can be decreased.

In some embodiments, the doping concentration of the N-type dopant in the current spreading layer is equal to or greater than 4×1018/cm3. In some embodiments, doping concentration of Se is equal to or greater than 6×1018/cm3. If the N-type dopant in the current spreading layer is noticeably less than 4×1018/cm3or doping concentration of Se is noticeably less than 6×1018/cm3, the carrier amount provided by the current spreading layer may be insufficient, and thus the current spread ability of the current spreading layer may be poor.

FIG.7shows the L-I-V curves for Embodiment 1 and the control group. Both Embodiment 1 and the control group are 940 nm VCSEL array, each having 85 emitters. The distance between any two adjacent emitters (center to center) is about 40 μm, and the bottom DBR layer is composed of GaAs high refractive index layer and AlGaAs low refractive index layer. The current and power conversion efficiency inFIG.7are the current and power conversion efficiency of each emitter, which are the average values of total 85 emitters.

The difference between Embodiment 1 and the control group is that the control group didn't have a current spreading layer in the bottom epitaxial layer, while Embodiment 1 had a current spreading layer disposed in the bottom DBR layer (seeFIG.1). The current spreading layer of Embodiment 1 is doped with Si at a doping concentration of 5×1018/cm3.

As can be seen fromFIG.7, both Embodiment 1 and the control group have the same output power, but Embodiment 1 has a lower operating voltage, indicating that the resistance of the lower DBR layer is lower. Therefore, the power conversion efficiency of Embodiment 1 is significantly better than that of the control group.

In the exemplary embodiments of the current spreading layer being directly or indirectly on the substrate, as illustrated inFIG.4orFIG.5, the current spreading layer can be a quantum well structure. Preferably, the quantum well structure is a stress compensation quantum well structure.